Dr. Hu is a questionable doctor who provides Sarah Lynn with drugs, going against the ethics of his profession and confuses BoJack with his name, as BoJack was thinking more along the lines of the popular British show and franchise Doctor Who.
Little is known about Dr. Allen other than his medical practice and that he deals drugs through a Black Market.
Dr. Hu is a human man with short brown hair, thick brown eyebrows bags under his eyes.
He is seen wearing a white button-down lab coat with two front pockets, a teal button-down collared shirt, grey necktie, teal jeans, and brown cowboy boots with a slight heel and a pair of large thin-framed glasses. He carries a large dark maroon medical bag with a dark grey strap.
Due to his name and his Black Market drug dealings, BoJack first suspects that his name is a pseudonym, prior to meeting the doctor.
BoJack is left speechless when the real Dr.Hu, not Who, makes his appearance.
Sarah Lynn, Todd, and the doctor himself don't seem to understand that BoJack is thinking of the popular British show Dr. Who and is getting confused by the similar-sounding name and don't make the connection.
He make a reappearance in Ancient History.
BoJack explains to Hollyhock that this doctor is the sleaziest and shadiest when it comes to drugs when they go looking for pills for BoJack's back after Hollyhock dumps them down the drain because she is paranoid BoJack is trying to feed her pills after what happened with Beatrice.
The doctor in question is Dr. Allen Hu. Dr. Hu reveals to BoJack after Sarah Lynn's death he got sober at Pastiches Malibu and became a brand new pediatrician and has turned over a new leaf . BoJack asks him for more drugs, but Dr. Hu refuses, due to the nature of the drugs he wants, and doesn't believe the story about BoJack's back.
It's also revealed he was an attendee at Sarah Lynn's funeral. It also seems, like he is trying to make BoJack feel guilty for not trying to change after her death, through their interactions.