I am one of the 6 people who waited for the official US release of Advent Children to get a copy of the movie. I'm kind of a failure as a Final Fantasy VII fan that way. However, I did pre-order the movie a year or so before it was released, and this makes me special. Still, I waited until about a month after to update this section, because I'm kind of a failure as a webmistress too.
Much as in Final Fantasy VII, Reno serves as comic relief. His antics provide much needed escape from the angsty world of Cloud and his Soul Wolf. He also has a very large role for being a minor villain in the original game. He has more lines than about half the playable cast combined, and that's not just because he won't shut up. Another noteworthy thing is that he spends all of his time on the side of the "good guys". This is because the Shin-Ra company has reformed. We now get to see the happier, fluffier side of Shin-Ra, and Reno's personality has been altered accordingly.
As to the plot- wait, why are you watching this movie for the plot? I, despite what this site may lead you to believe, am a hardcore Final Fantasy VII fanatic. I read the prequel novels and replayed the game before watching Advent Children, and it still didn't make very much sense. It's very pretty, and that's all that matters. Really, Nomura told me so.
During Advent Children, Reno is loyal to bizzarrely-not-WEAPONED Rufus and his fellow Turks, including the bizzarely-not-encapsuled-in-Black-Materia Tseng. (Kiss that promotion goodbye!) However, Rufus has changed his tune to the song of strategically placed blankets, and likewise to the white and fuzzy side of the force. He's trying to stop the new evil of Silver Haired Men/JENOVA: The Boyband!, who are out for JENOVA's head, the resurrection of Sephiroth, and all things generally evil. Rufus wants the Turks to join with the hero party to stop the Silver Menace, and Reno seems happy to oblige. (Hey, who hasn't wanted the oppurtunity to kick Yazoo in the face?)
And that's my summary of Advent Children. Oh, the good guys triumph and Reno gets to save small cuddly children.
Seriously, Reno acts as ally to Cloud and Co, and seems to get along well with Tifa in particular,
despite having personally dropped the plate on Midgar because he's such a fun guy. However, Reno does his fighting with Rude away from the AVALANCHE action. The Reno/Rude fight sequences are more slapstick and less "ZOMG DID HE JUST THROW A MOTORCYCLE WITH HIS LEGS?!" but they are able to keep, uh, Loz and Kazoo, at bay while Kadaj gets his wangsty butt beaten by Rufus. They then prove they're worth a damn by blowing enemy motorcycles up with flashy explosives, chuckling about overtime. Thus they combine ineptitude with equal degrees of badass that is the Turk way.
Oh yeah: Keep on rockin' in Midgar is the best thing ever put on a monument, fictional or otherwise. Shin-Ra, you're my favorite bunch of
extremely unlikely do-gooders.