Reno, having been with the company for a fairly long period of time, has gone through a lot and is quite possibly one of the most complex Turks in the FFVII compilation. Since the beginning, he has had a similar look, with the bright red hair, generally disheveled looking suit, and slouching pose-- all signs of his rebellious streak. In Before Crisis, he's also seen as energetic and impatient, preferring, as he says in his secret episode, the flashy jobs to the dull, paper-pushing ones. In the same episode (one that includes missing files and also out-of-control robots), he's seen complaining to Rude when Tseng and Veld assign them to keep an eye out for suspicious persons in the archives. Rude, in turn, tells him that even the boring parts are his job and to work harder (which Reno just brushes off). When the real action starts (crazy robots attacking), he brightens up and goes about destroying them with true enthusiasm and confidently proclaiming he'd be able to dispatch them in a short period of time. He also shows his reckless side, diving into the fight headfirst without asking too many questions.
As the mission goes on, there's more of his cockiness, recklessness, and displeasure with what he finds to be "boring" parts of the job. Some of the carelessness shows another side to him, however, when he's chasing down a possible perpetrator into an elevator shaft as the elevator descends. He shows his tenacity to continually pursue a target and to think very clearly when he's in pretty serious danger, especially when the elevator starts ascending and he'll die if he can't get the hatch open on the lift. During this scene, he really relies on his physical speed and his mental speed, knowing he'll be able to clear any obstacles with it. Moreover, he places all of his trust in his teammates, Veld and Tseng, to get him the elevator hatch codes in time. He's trusts them with his life and doesn't doubt them for a second.
In other missions later on in BC, his more amusing, joking side is shown, especially when he's on the job with Rude. This is demonstrated in one particular episode, when he, Rude, and Tseng are going off to locate Veld, they end up in a trashyard/junkyard, which sets Reno off on spouting some puns with Rude about intruders being taken out with the trash. He clearly likes to enjoy himself while working, probably because he puts in enough hours, may as well have a laugh every now and again.
When the original game starts, though, some of the qualities he had in BC have changed to be more subtle or nonexistent. Many of his teammates in BC are (supposedly) dead, which is most likely a huge factor in him mellowing out. Also, during the Nibelheim incident with Sephiroth massacring the town, Reno and his teammates are ordered to take the town's survivors into the Shinra Manor, which would be placing them, essentially, into Hojo's unsavory hands. Up until this point, Reno hadn't really questioned what had been asked of him, but this was a dirtier job than he'd been expecting-- ultimately, Veld covers for his Turks so they don't have to do it. Continuing his job despite these more questionable things coming up has sobered him up, and in FFVII, he's less reckless and enthusiastic as he had once been, instead seeming more jaded, cruel, and nonchalant. He's grown used to the shady things he's asked to do, and is rarely ruffled by them. The biggest example of this change is seen when he drops the plate on Sector 7, not even blinking as he does, and even seeming to bitterly mock the whole situation. Also, the few times he encounters Cloud and Co., this sort of bizarre apathy can be seen, since he never seems to really put all of himself into trying to apprehend the group (in game, the last fight with the Turks is completely optional, as Reno himself says he's "not up for" fighting).
In general, Reno seems to be sticking with Shinra not for the company, but more for his team. He's still loyal to them and only to them, since the rest haven't earned it. This kind of loyalty to his team and apathy towards Cloud's mission is really demonstrated when Elena and Yuffie are captured by Don Corneo in Wutai. The only reason he fights Don there is because he wants to save his teammate and he takes pride in upholding the reputation of the Turks. He doesn't even bother to try and capture Cloud and his team, despite the company having given him orders to-- he waves it off, saying he's not working since it's his day off.
Reno's character goes through some really rough changes from BC to FFVII, especially since he's forced to do some morally questionable things in the original game (it is mentioned, though, in BC, that he and the Turks help to evacuate certain sectors of people during Meteorfall during FFVII). However, in Advent Children, some of his original brightness, enthusiasm, and relatively good personality can be seen coming back. He still bears no ill-will towards Cloud, fighting him half-heartedly in Healen Lodge, and even helping him and Tifa out later in Aeris' church, bringing the two of them to 7th Heaven to recover while they're unconscious. Later, he shows up with Rude to help out Cloud and Co. when Bahamut is in Edge, all snark and confident swagger. He dives into this fight with a recklessness that hearkens back to his BC days, seemingly back to his normal self again (just with a bit more experience under his belt). Lastly, he attempts to take care of Yazoo and Loz with Rude, by way of explosives, to help out until Cloud shows up.
Overall, he's not a completely bad guy, and in AC, even shows a pretty good side in helping out Cloud to overcome the Silver Haired Trio + Geostigma. His aid could be seen as somewhat selfish, as Rufus is afflicted with the stigma, and by that point in the FFVII compilation, the blonde had been accepted as an honorary Turk (and kind of their Boss), which Reno honors with his loyalty. He does have a darker, more jaded side to him, though, which is clearly demonstrated in the original game. The situations in AC dictate that he and the Turks don't need to be as morally questionable/cruel, and therefore, he's a bit more like how he was in BC.
Re: PENDING
As the mission goes on, there's more of his cockiness, recklessness, and displeasure with what he finds to be "boring" parts of the job. Some of the carelessness shows another side to him, however, when he's chasing down a possible perpetrator into an elevator shaft as the elevator descends. He shows his tenacity to continually pursue a target and to think very clearly when he's in pretty serious danger, especially when the elevator starts ascending and he'll die if he can't get the hatch open on the lift. During this scene, he really relies on his physical speed and his mental speed, knowing he'll be able to clear any obstacles with it. Moreover, he places all of his trust in his teammates, Veld and Tseng, to get him the elevator hatch codes in time. He's trusts them with his life and doesn't doubt them for a second.
In other missions later on in BC, his more amusing, joking side is shown, especially when he's on the job with Rude. This is demonstrated in one particular episode, when he, Rude, and Tseng are going off to locate Veld, they end up in a trashyard/junkyard, which sets Reno off on spouting some puns with Rude about intruders being taken out with the trash. He clearly likes to enjoy himself while working, probably because he puts in enough hours, may as well have a laugh every now and again.
When the original game starts, though, some of the qualities he had in BC have changed to be more subtle or nonexistent. Many of his teammates in BC are (supposedly) dead, which is most likely a huge factor in him mellowing out. Also, during the Nibelheim incident with Sephiroth massacring the town, Reno and his teammates are ordered to take the town's survivors into the Shinra Manor, which would be placing them, essentially, into Hojo's unsavory hands. Up until this point, Reno hadn't really questioned what had been asked of him, but this was a dirtier job than he'd been expecting-- ultimately, Veld covers for his Turks so they don't have to do it. Continuing his job despite these more questionable things coming up has sobered him up, and in FFVII, he's less reckless and enthusiastic as he had once been, instead seeming more jaded, cruel, and nonchalant. He's grown used to the shady things he's asked to do, and is rarely ruffled by them. The biggest example of this change is seen when he drops the plate on Sector 7, not even blinking as he does, and even seeming to bitterly mock the whole situation. Also, the few times he encounters Cloud and Co., this sort of bizarre apathy can be seen, since he never seems to really put all of himself into trying to apprehend the group (in game, the last fight with the Turks is completely optional, as Reno himself says he's "not up for" fighting).
In general, Reno seems to be sticking with Shinra not for the company, but more for his team. He's still loyal to them and only to them, since the rest haven't earned it. This kind of loyalty to his team and apathy towards Cloud's mission is really demonstrated when Elena and Yuffie are captured by Don Corneo in Wutai. The only reason he fights Don there is because he wants to save his teammate and he takes pride in upholding the reputation of the Turks. He doesn't even bother to try and capture Cloud and his team, despite the company having given him orders to-- he waves it off, saying he's not working since it's his day off.
Reno's character goes through some really rough changes from BC to FFVII, especially since he's forced to do some morally questionable things in the original game (it is mentioned, though, in BC, that he and the Turks help to evacuate certain sectors of people during Meteorfall during FFVII). However, in Advent Children, some of his original brightness, enthusiasm, and relatively good personality can be seen coming back. He still bears no ill-will towards Cloud, fighting him half-heartedly in Healen Lodge, and even helping him and Tifa out later in Aeris' church, bringing the two of them to 7th Heaven to recover while they're unconscious. Later, he shows up with Rude to help out Cloud and Co. when Bahamut is in Edge, all snark and confident swagger. He dives into this fight with a recklessness that hearkens back to his BC days, seemingly back to his normal self again (just with a bit more experience under his belt). Lastly, he attempts to take care of Yazoo and Loz with Rude, by way of explosives, to help out until Cloud shows up.
Overall, he's not a completely bad guy, and in AC, even shows a pretty good side in helping out Cloud to overcome the Silver Haired Trio + Geostigma. His aid could be seen as somewhat selfish, as Rufus is afflicted with the stigma, and by that point in the FFVII compilation, the blonde had been accepted as an honorary Turk (and kind of their Boss), which Reno honors with his loyalty. He does have a darker, more jaded side to him, though, which is clearly demonstrated in the original game. The situations in AC dictate that he and the Turks don't need to be as morally questionable/cruel, and therefore, he's a bit more like how he was in BC.