Sensei is a Japanese word which means "Someone's elder" and is often used as an honorific for teachers.
In the Warhammer 40,000 universe, the Sensei are supposedly the immortal offspring of the God-Emperor of Mankind, left over from the time when he was walking around and living as a man. In the good ole' days of the fluff, their end goal was to achieve apotheosis and merge their souls with the fragment of the Emperor floating around in the warp - the Star Child. They were also all blanks.
In fairness, the lore change does make a certain amount of sense. Sensei were introduced in the Realms of Chaos duology, specifically in "The Lost & The Damned", and right from the beginning they were outright compared to Good-aligned Chaos Champions, to the extent that not only were they playable as a variation of the Path to Glory game, they had their own Chaos Mark equivalent and even fought to attain Apotheosis, a transformation into a Warp-entity called a Sensei Master that was literally called out as a Daemon Prince of the Star Child!
Playing a Sensei
In the "Path to Glory" type game that they were designed for, Sensei "armies" - Adventurer Bands, they were called - were small and elite. Instead of gaining large numbers of mooks with a smattering of exceptional individuals like a Chaos Champion, a Sensei picks up long Character-level individuals each time its band increases. As a result, a Sensei's party lacks numbers, but each member is a powerhouse compared to the average trooper they'll be going up against.
As "Champions of Good", Sensei had their own unique array of mystical abilities and powers they could pick up as they fought on and evolved through the Path to Glory.
To start with, the "Mark of the Star Child" grants a Sensei a +d6 bonus to Strength, which is rerolled each time a Strength check is made, such as when making hand-to-hand combat roles.
The Star Child Rewards table is much smaller than that of any of the Chaos Gods, but aside from one or two rewards that push you towards acting like a goody-goody, they also fuck the Sensei over way less both in general and in specific instances. Like Chaos Champions, a Sensei can "pass on" Rewards to members of their force, making them stronger to represent the growing power of the Star Child cleansing their allies and making them more sublime.
Protector: Acquired on a 01-10 result, this grants the Sensei and any ally within 6" the ability to reroll a failed save (of any kind) once.
Daemon Slayer: Acquired on an 11-15 result, the Sensei's hand-to-hand combat attacks ignore daemonic resistance and cause d6 Wounds per hit on daemons.
Sword Master: Acquired on a roll of a 16-30, this grants the Sensei a +1 bonus to Weapon Skill, Attacks and Initiative. None of these traits can be increased above 10 by this reward.
Marksman: Acquired on a roll of a 31-45, this grants the Sensei +1 Ballistic Skill and causes the maximum range of the Sensei's firearms to increase by +10%. Ballistic Skill bonus maxes out at 10 and the Sensei can only double the maximum range of any weapon.
Endurance: Acquired on a roll of a 46-55, the Sensei gains +1 Wound and then rolls a D6; on a 4+, they gain a second Wound! Oh, and there's no limit to the number of bonus wounds that this Reward can grant!
Athletic: Acquired on a roll of a 56-60, the Sensei's Charge rate triples their movement value and the Sensei now ignores any movement penalties caused by hand-held heavy weapons.
Rescuer: Acquired on a roll of a 61-65, the Sensei must always attempt to go to the aid of any ally who is designated a casualty within 3". This is done by rolling a D6. On a 1 the Sensei fails and is so grief-stricken that they cannot move, shoot or attack until the end of their next turn. On a 2, the Sensei succeeds, but is rendered vulnerable; no moving, shooting or attacking until the end of their next turn as they seize the initiative back. On a 3+, the save is pulled off without a hitch.
Never Kills: Acquired on a roll of a 66-75, this means that a Sensei always fights to spare a living opponent (daemons, undead, and other unliving monstrosities don't count!) - this has no effect in actual play, but in a campaign, any model taken out of action by the Sensei automatically recovers after the fight.
Heroic Name: Acquired on a roll of a 76-80, this just gives the Sensei a badass name. It counts as a reward for attain Apotheosis, but has no actual game effects.
Master of Disguise: Acquired on a roll of an 81-85, the Sensei gains the power to disguise himself an ordinary human model until either the player chooses to reveal it or it is forced to act by the Rescuer reward.
Redeemer: Acquired on a roll of an 86-90, when the Sensei defeats an enemy hero, if they choose to spare their foe (or must do so due to Never Kills), there is a chance that the foe will be tempted to join the Sensei's party! Both models roll a D6, with Chaos Champions doubling their result; if the Sensei's result is higher, than the hero joins the Sensei's party. Redeemed Chaos Champions renounce their Patron and lose any "disfiguring" Chaos Attributes, but may potentially retain more mystical or subtle powers, which are now fueled by the grace of the Star Child.
Apotheosis: Acquired on a roll of a 91-00, the Sensei has received the direct attention of the Star Child, and must check their tally of Rewards. If they have 6 or more Rewards, they become a Sensei Master and vanish into the Warp. If they have less than 6 Rewards, they gain +1 Attack and +1 Wound.