Going by past history, a sycophant is a must. Someone who will approve of nearly any action you take, but never confront you on too personal a level for the one or two things they personally dislike. Frequent words of praise recognize the inherit properness of the player, emotional support of how 'hard' or 'difficult' a luxury entertainment game plot is on the psyche of the player(character) assists in immersion, and basking in shared glory by association makes the sycophant appear capable. A frequent longing to be accompanying the player character for 'fun' rather than pragmatic reasons is also encouraged.
Ideological consistency, however, is not important. Ideological flipflops will be ignored in and out of the medium, as consistency is the first casualty of fawning (and forcing the character into the most convenient plot position). The BFF's ideology should mirror the player's, no matter what the player's is, and no matter how often the player flips. The Best Friend will never challenge the player on any perceived inconsistency.
Casual cruelty to unsympathetic characters is more than acceptable- it's encouraged, as it shows someone is a 'badass' and is a daring way to not-really challenge the 'goody two-shoes' moralism if the character has a 'dangerous' side that only gets unleashed on acceptable targets. Acceptable targets include rhetorical bigots, opposing mercenaries, or any mouthy civilians who don't get with the program and support the sympathetic characters.
Fourth, the character needs to have little life outside of the protagonist, or at least no life that either doesn't conveniently support the protagonist's plot or won't be freely abandoned after a recruitment quest. The character's focus on supporting the main character ably, while being unable to keep down a job outside of being the protagonist's lackey is a charming testement to their dedication.
Finally, the character must never confront the player on any personal and direct level that can't simply be blamed on the character acting on a heat of the moment. Any reaction should preferably be aimed at other characters, but most of all the BFF should be appropriately contrite for any behavior towards the player, no matter how minor, and equally grateful for the benevolence and wisdom of whatever the player chose in the confrontation scene. This sort of character 'conflict' is acceptable because it proves that the BFF 'gets it,' understands their place in subordination to the player, and ultimately revers the centrality of the player while never challenging them on a personal level by assuming all blame themselves for any heretical difference of opinion.
And really, isn't that what a good Best Friend in a game should do?
Follow these simple steps,without actually describing them, and you to can write a fan favorite character that will have people with unrealistic expectations wishing they had a real-life sycophant friend and writing dozens of threads wishing they could ****** to you character as a romance!