Not much is known about Leader's appearance because he wears a masked costume. Therefore, we do not know his species. He wears a full domino mask all over his head as well as a costume. This is the exact outfit that the rest of the asssassins wear, except that Leader's masked eyes are red instead of white.
Not much is known about Leader, be he has been the leader of The Master Assassins for a long time.
Ben 10 & The Master Assassins
In Ben 10 & The Master Assassins, Leader is the main villain of the series as he leads a new group known as The Master Assassins. They assassinate their targets in order to bring justice, or what they believe justice is.
In Enter Assassins, Leader is shown outside the target warehouse where he confronts Ben. He tricks Ben and transports out. He later breaks into the Plumber's base and he murders Magister Patelliday, ultimately ruining Ben's confidence.
In The Dudesman Theory, Leader tricks Ben into going into a fight in Undertown. He then appears to kill Rad Dudesman in front of Ben, crushing his confidence even more. However, it is revealed at the end of the episode that Rad actually escaped before Ben and co even arrived, and Leader only tricked Ben into thinking he's dead to destroy his confidence.
In The Hunt For The Illuminati, Leader kidnaps Blukic and Driba as they know their next target. He was originally planning to kill them, but an assassin convinces him to keep them as scientists. Leader is convinced but he wants to keep it a secret, so he kills the assassin.
In Breaking News, Leader kills Will Harangue using a sniper rifle. He then announces that everyone they targetted until now is to crush Be's confidence, and that the real targets will start with their next one, Max Tennyson.
In Real Targets, Leader leaves the assassins to kill Max Tennyson on their own, and they fail. Meanwhile, Leader goes to confront Rook Blonko. He does not want to kill him, but he offers him a place with The Master Assassins. At the end of the episode, Rook is shown contacting Leader.
In Right On Time, Leader orders his mole, Rook, to kill Professor Paradox, which he does. Rook then escapes, as ordered by Leader. Leader says this is to manipulate Ben into thinking Rook has been kidnapped while they all go into space after Azmuth.
In Guilty As Charged, Leader kills Azmuth. It is also revealed that he is not the first leader, and Azmuth was the first leader. Leader also reveals that the final phase of the plan has started.
Before he kills a victim, he would always yell " *Victim name here*, you have failed this universe". This is a reference to the TV show Arrow, where the main character yelled this before killing people during the start of season 1. However, instead of universe, he said city.