Nigel is the most charismatic nerd in your party, or at least that's what his mother says. He's specialized in casting powerful area spells from the back of the formation and shielding himself from damage, so don't expect him to withstand a lot of punishment.
With Nigel as a leader you can boost you party spirit and/or mp pool.
Main is Charisma
One key tactical consideration: If you make sure that Mike is faster than Nigel and has the strongest Spirit-boosting skill he can find, and uses it on Nigel on his first turn, Nigel effectively turns into a walking artillery unit. However, also be warned that this generates a lot of Hate for Nigel and unless Johnny intervenes with one of his Hate-generating skills every enemy on the field will want revenge on Nigel for his attacks.