Who, Dodonpa's child or Isaac? If the former, no; he's just a minor supporting character whom helps Matthew now and then. If the latter, no; at least not right away. After defeating Isaac, he is much too weakened to be of any use to the party, so with his last bit of strength he retreats after giving Matthew and crew some helpful advice and passing Matthew some of his power. Similar to Riku in
Kingdom Hearts 2 and Athos in
Fire Emblem: Blazing Sword, Isaac is one of those insanely powerful characters whom you don't get to recruit until well near the end of the game.
As for new psynergy, I thought of a new type of psynergy called "Golden Psynergy", which is exclusive to Isaac, Matthew, and Alex/Arcanus. You could call it the counterpart of Dark Psynergy. Golden Psynergy can ONLY be accessed after entering a special state called "Dawn Mode". Once you're in Dawn Mode, for a limited amount of turns you have an infinite amount of Psynergy Points (PP), and can cast godly-powerful psynergy moves (one of them being Cruel Ruin
). You are also given massive stat boosts--how high each stat increases depends on which Djinn you have equipped, and how many are set.
BUT, as a price, once Dawn Mode has ran out of time, your PP will be reduced to zero, and all off the caster's Djinn will enter recovery mode upon activating Dawn Mode--and the Djinn won't begin to reset until after Dawn Mode is over. Also, for one turn, the character whom activated Dawn Mode must recover (similar to Hyper Beam in the Pokemon games). Just like
Dark Mode, this is a high-risk, high-reward battle mechanic. Lastly, to prevent abuse, Dawn Mode can
only be activated
once per battle.
[Shrugs] Maybe not the best idea, but I tried to make it complex, whilst at the same time easy to understand and pick up.