Mortarion

Mortarion was one of the founding members of the crusade, in fact it was he who first planned it. although he does not enjoy a position of de facto leadership like Dorn and mostly just argues with people over whether or not to do some morally reprehensible action, or about intentionally poisoning food.

having returned from the eye of terror he appears to have a somewhat fractured mental state, now exclusively using the number 7 and having fits of hysteria where he believes he is back on Barbarus. he has also commented on pre-existing mental problems such as his all consuming inferiority complex, extreme prejudice towards sorcery (the word witch gets thrown around a lot) and a general strange set of ethics, believing necromancy to be completely abhorrent but forced chemical sterilisation and mass genocide to be no worse than swatting a fly.

eldar origins
it was revealed relatively early in the crusade that Mortarion was formed form the emperors DNA and some of Isha's (later revealed to come from a tear) to prove this the emperor healed Mortarion's ears which had been bitten off by kill beasts as an infant: to reveal that they were pointed like those of an eldar. following the identity crisis that came with the revelation he accepted his origins, although to the surprise of none, despite hailing fro the most powerful race of psykers still detested the warp. he has also reported being able to hear emotions with his ears, often using this to support his arguments to forcibly sterilise magnolia

the number 7
Mortarion has developed an obsession with the number 7, using it exclusively and utilising overcomplicated formulas involving too many sets of brackets to say simple numbers. the number seems to comfort him, bringing him down from his hysterical episodes and giving him strength, in contrast he loathes all other numbers, calling them inferior and wretched, seeming distressed from their usage and even launching into a manic episode when they are defended.

the number 7 appears to have great power when used by him, he keeps Necare and a sister of a battle who touched his ears within the pain clock using nothing but 7's, and claims to have defeated a legion of chorine using nothing but the number 7 (presumably for the great game within warp, where he is much more powerful)

the pain clock
Mortarion is in possession of a large grandfather clock which he refers to as the pain clock, it uses nothing but poison and the number seven to bind and torture the souls of the dead, which currently include his abusive foster father Necare and an unnamed sister of battle who fondled his ears. This apparently functions using nothing but the number 7, and as a result will release all captured souls should any number the than seven be spoken near it, interestingly enough when the souls are freed they are given new flesh, but for some reason nobody comments on this