I haven't read as much Kevin Keller as you have, but I feel he's been very well characterized in the stories I've read. I like the idea of a pretty boy foil to Jughead who has the complete opposite reaction to Veronica that Archie does. Lots of rich inversion of classic Archie tropes. There's ample material there without even getting into the fact that he's gay.

Moose is one note. Veronica is slightly more complex, though she is more or less pretty one note as well. Jughead is very complex, more so than any of the other Archie character, though he has certainly been reduced to at least two notes on many, many occasions (writers who rely too heavily on "loves to eat" and "hates girls" as the Jughead's only characteristics get an instant thumbs down from me). 

One note is fine for Harper if she's just going to be a one off character. If she's going to be a permanent addition to the cast, she needs to bring a little more, as the established cast all have defined story roles they fill. The problem I have with Kevin in the regular comics - not Life or Afterlife - is that they just seemed to throw everything at the wall in an attempt to find something that stuck with him, which usually meant just having him take someone else's role over. Things like having him out-prank Reggie and beat Jughead in an eating contest - it's the Archie equivalent of having Cannonball defeat Gladiator to prove he's worthy of being promoted to the X-Men.