Are you guys debating between a 6 and 7 for Guy?
8. Easily.
With the exception for Marcus and possibly Hector (depends on where he's going since he seizes), Guy is your earlygame offensive machine. 17 Atk with the Killing Edge along with 36 Crit at base level (level 3, mind you) is absolutely nuts. 6 Def 15% growth is obviously terrible but it is backed up by 25 HP and 75% growth which can leave him with 38 HP and 8~9 Def by the time he hits 20/0. That's certainly manageable since he was created to be a dodge tank. That Matthew support is easier than it looks to build since Guy will have 2~3 turns at least in his joining chapter, possibly up to 7 in 13x, possibly 5 or 6 in 14 (both work over the pirates from the southeast) and be Matthew's guard in 15, giving another 3 or so since Matthew will die otherwise. That's a C before Lyn's team even joins and it is perfectly conceivable.
Guy can be clutch if you can use him right. If you toss him into 10 enemies and expect him to survive ala Raven style, then he's definitely got a chance to die.