How would a Knight of Doom and Thief of heart interact in a standard session?
99% depends on the personalities of those involved
One thief of heart might be somebody who takes the personas of other people to alter themselves, a malicious being who tries to piggyback on the knight of doom’s natural ability to exploit failure and death to bring them more souls
Maybe the knight of doom would be a stoic but ultimately single-minded warrior, always making sure that no death or failure goes without a lesson or without an advantage. So the knight of doom, coming across this malicious thief of heart, would seek to purify them and the aborated souls they possess
This thief of heart might copy the attributes of others to benefit themselves, to become the best possible person they can be. Taking in solitude and creating golems in their likeness to search out virtuous traits in others, so that they can improve themselves and reach enlightenment.
And maybe this knight of doom is an erratic and frayed individual. Maybe this knight of doom enjoys slaying others and causing destruction for no reason other than personal enjoyment. When this thief of heart comes across this knight of doom, maybe the thief will be angered by the knight’s disregard for life. this thief is unable to take attributes from the dead. so the thief may call upon their golem-selves to destroy the knight of doom, to prevent them from spreading any more destruction
SoInteractions between different titles really depend on the personalities of those involved more than the titles themselves