Finn and JakeEdit
Lee was rescued by Finn and Jake. Lee might have known them ever since, although he hasn't been seen in previous episodes. Lee first is seen with Finn and Jake in BMO BRO where he repairs the Brobot for Finn and Jake and helps them again when the Brobot had gone completely insane. Finn and Jake seem to known Lee from before but that hasn't been on-screen.
Lee helps Princess Bubblegum on "BMO BRO" to recharge up Brobot with his Electro Powers. Princess Bubblegum knew about Lee's existance in Ooo, but if it is because they met before or duo PB knowing everyone in and close to her kingdom is unknown.
Lee and BMO have really bonded because BMO is a machine and Lee is interested and handy with technology. He used BMO to connect in "BMO Classic Collection" and was in the same team as him to save Princess Bubblegum. In "BMO BRO" he saved BMO from the evil MO that he created himself.
Gene is Lee's best friend.
Lee and Death are long time enemies since Oil Slip.
Lee and Ice King seem to be neutral enemies.
Anti - VirusEdit
On The Anti - Virus, Anti - Virus becomes Lee's second arch - nemesis,