Relationship with Friends

Finn and Jake
BMO lives in the Tree Fort along with Finn and Jake and is used as their game console and many other things. It is loyal to Finn, as it said it will kill anyone who tries to hurt Finn, as seen in "Incendium." Also seen in "Video Makers," BMO does not side with either Finn or Jake. BMO cries when Finn shaves off his hair to be "Davey," sobbing that his "beauty" is destroyed.

Marceline
It seems that Marceline and BMO have a fairly friendly relationship, as she is seen playing it in "Evicted!" and commenting on how she thinks it is cute. Also in "What Was Missing," Marceline assists BMO and proceeds to call it "baby." 

Lorraine

In "BMO Noire," BMO mentions that Lorraine is "red hot like pizza supper." Furthermore, Lorraine also mentions that the loot is in their "secret grown up kissing spot" at the end of the episode, thus implying that at one point in time they had a relationship. This might be taken as an indication that BMO finds Lorraine attractive. However, the entire episode hints that the plot of "BMO Noire" is nothing but a play created by BMO's imagination. So, it is possible that their relationship exists only in BMO's fantasy. It is also possible that BMO is not attracted to Lorraine and the attraction/relationship is fictional like the detective persona that BMO created for itself.

Football
Football is BMO's reflection in the mirror or window or whatever it can see its reflection in. BMO tells Football about all of the things in the real world and tries to educate Football on how to act and what things are in the world so that when Football grows up she will know what to do. BMO does this to keep it from feeling lonely whenever Finn and Jake are away. Football appears in the "graybles" episodes, "Five Short Graybles" and "Five More Short Graybles." Air (Bubble)

 

Air (Bubble)
Air (Bubble) is a pal that BMO met in the episode "BMO Lost." After an adventure to help BMO get home through hard times with a baby named Sparkle (whom BMO had named Ricky), Bubble confesses that it is in love with BMO and proposes they get married, which BMO gladly accepts. Sadly, Jake decided to pop the bubble, making it now Air. 

Sparkle (Ricky)

Sparkle (Ricky) appears in the episode "BMO Lost." This baby was lost and separated from his mother, so BMO decides to name the baby "Ricky" and bring him along on the adventure of all three characters, BMO, Air (Bubble), and Sparkle (Ricky), trying to find their home. Sparkle is later rediscovered by his mother and taken to his rightful home.

 

Moe
Moe is BMO's creator, as shown in the episode "Be More." He created BMO because he wanted BMO to take care of his son, but he "never ended up dating any women." Moe created BMO so it could "Be MOre."