Zelnite sounds pretty OP...
Some thoughts on Aem vs Leorone as leaders (+3 BC every turn vs +damage and more BCs during sparks)
I have a maxed Aem that I've been running with an unlocked SBB, but his stats are pretty poor. He's not that terrible though since his SBB increases the drop chance of BCs. So he can fill everyone back up if it only takes <30 BCs to fill their BB. If I'm running with Felneus, Aem's SBB can even refill Gravion's BB which takes like 46 BCs to fill.
To try something different, I powered leveled my Leorone to try him out as leader, and it's different, but maybe not better. It's not that difficult to stack a team for sparks, and when they do go off, it's a pretty noticeable difference in damage. However, clearing trash is definitely slower since I can't just spam all my BBs to kill them in one round and be ready for the next. Sparking works best single target, and switching between a lot of trash minions lowers the overall damage. Damage on bosses though is more consistent with Leorone as lead vs Aem. With Aem, I'd save up my BBs/SBBs and spam everything at the boss. For whatever reason, BC generation isn't that great on bosses, so I'll limp along for a few turns waiting for things to refill. With Leorone lead, I can still get really good damage from normal attacks during sparks and the boss battle will be shorter overall.
TLDR version: Aem = great burst damage, with his SBB refilling BBs really quick on trash, but not on bosses. Leorone = great sustained damage, trash takes longer, but bosses are quicker