Regis thinks of himself as the right-hand man of Papa Khan. This is also literal, as Regis takes the seat to Papa's right in the Long House. The bed he sleeps in is surrounded by pre-War books as well as a few magazines, suggesting that Regis is well-read and educated. Regis is also the enforcer of all the Great Khans laws and as such will warn the player to watch their step.
Oh My Papa: Regis is not convinced that the NCR's actions at Bitter Springs were committed intentionally. If you help 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 becomes Papa Khans 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 player 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.