Regis thinks of himself as the Papa Khan's right hand man. This is also literal, as Regis takes the seat to Papa's right in the Long House. Regis is also the Great Khan's law enforcer and as such will warn the Courier to watch their step.
Oh My Papa: Regis is not convinced that the NCR's actions at Bitter Springs were committed intentionally. If the Courier helps Regis usurp control of the Great Khans from Papa Khan, he will send his forces as aid to the NCR during the Second Battle of Hoover Dam.
We Are Legion: After the player character becomes Papa Khan's heir, it is possible for them to quietly kill Papa Khan, take his Great Khan spike helmet, and then become the new Papa Khan. Speaking to Regis after this transition (which is instantaneous) will allow a Legion player to decide the fate of the tribe.
Regis will not offer the quest Oh My Papa if the Courier has earned a vilified reputation with the Great Khans, stating that he "should not be talking about this with an outsider." This can be fixed by picking up Karl's journal, talking to Papa again and leaving the longhouse.