Philip Seymour Horseman

Has a long face, like a horse, man.
The name "Philip Seymour Horseman" was alluded to many times throughout the Space Tree series. It was kind of a running gag that characters would always mention this character who we never see or know anything about.
References to Philip Seymour Horseman included:
* In Panic Broom, the Commander had a list of "People I Know". It included Philip Seymour Horseman.
* In Serious Tingling, Space Tree complained before he was about to die: "I mean not even Philip Seymour Horseman showed up."
* In The Giving Tree, Tarbob mentioned his upcoming interview Philip Seymour Horseman on Chanel K News.
* In The S Word, Philip Horseman is one of the names Mee lists when trying to guess Allon's real name.
* In The Italian Job, PinkFrosting88 and Reginald watched a film called Revenge of the Long-Face Man starring Philip S. Horseman as The Long-Face Man.
* In The Tree Whisperer, the Commander tries to get into The Captains Club with a trial membership card to the unofficial Philip Horseman Fanclub.
* In The Sanctity, the Commander puts up a movie poster of "L for Longface", a movie starring Philip S. Horseman.
* In The Commander, Philip Seymour Horseman signed the mug for the "World's greatest Commander award". Tahoma also reveals here that he's the founder of Chanelle J News.
* In The End when the ship exploded, one of the pieces of the ship has "HORSEMAN" written on it.
We finally got to see Philip Seymour Horseman on screen in the in the episode Phil, where he plays himself as the titular 'Phil'. The episode is sort of a variety-show special with Phil as the host presenting clips related to Space Tree, similar to The Simpsons 138th Episode Spectacular where a character who exists within the show's universe breaks the fourth wall and presents clips from the show. The only character from the actual show who Phil appears in the same scene with is Mee. 
Philip Seymour Horseman is an obvious reference to Philip Seymour Hoffman, an Academy Award-winning American movie actor who is dead.