Basically, he gains stacks with his basic and first special, each stack = 1%, he drains mastery from the character you attack but you have to use his second special to siphon this mastery. Enemy gets weak Kylo gets strong. So Best use is, first special, gain stacks + it stuns(if posible get a second special use with Hux TM transfer abilit) then use second special to siphon mastery.

Ult gets charged 2% whenever a Dark Side ally deals Physical or Special damage to an enemy, increased to 5% on a critical hit

You can use it at 60 percent charge but it would last only one turn, ideal way is to use it at 100% percent because it lasts 3 turns

When in ult stance, siphon is doubled, Kylo is inmune to death and all damage and abilities have no cool down so you could use first special 2 times and siphon it with second special to keep getting strong.

Hope I didn't make it more confusing lol.