Both the Claimant and the current leader have good reason for supporting their right to rule.

Talking to the current leaders of each faction, you can ask for their side of the story. Such as: The Latin Empire's vassals are bad relations with Emperor Baldwin II of Courtenay which caused Emperor Angelo rise to power and become the Empire of Aegean. If you are not the side with the claimant, with a monarch in bad relations with it's vassals, they will turned against the monarch and joined the claimant side. Once joined a claimant's cause, the claimant will be the ruler of the rebel and will join player's party while player will become the first vassal and the marshal of the claimant's faction: ' Rebels'. The requirement of joining a claimant is the same as monarchs, player must have required enough renown first (at least 200 renown). Player is the first person who can decided whether to help the claimant or not.

Like becoming a vassal of a faction, the claimant also seek help from capable warriors. Siding with a Claimant Joining a Claimant's Cause

With fire and sword guide