Secret bosses are bosses that require specil conditions to be even be fought, and can be seen in a non-secret boss form.
The Shadow Mantle holder, ERAM, is probably not a secret boss. He requires Sword Route, yes; but he is only seen in the secret boss form.
Spamton NEO is a secret boss.
Jevil is a secret boss.
Gerson is a secret boss.
Mike might be a secret boss? I haven't done much of Chapter 4. I only recently beat Jackenstein and tried to fight the Hammer Of Justice.
Shadow Crystal bosses drop Shadow Crystals.
Spamton NEO is a Shadow crystal boss.
Jevil is a Shadow Crystal boss.
Gerson is a Shadow Crystal boss.
Can't confirm Mike being a Shadow Crystal boss, but I'd assume not since if you got C1, C2, C3, and both C4 crystals then you’d make the pure crystal which isn't possible yet.
The Roaring Knight is a Shadow Crystal boss.
Bonus bosses are completely optional and do not need to be fought or beaten to finish the Chapter.
Spamton NEO is a bonus boss.
ERAM is a bonus boss.
Mike should be a bonus boss, can't confirm.
The Knight is not a bonus boss due to being a forced fight, though one could make the argument that winning is optional.
Jevil is a bonus boss.
I could be missing a few, but these are all I know of.