At the end of "I Am a Sword," BMO says "I like Air" when Jake suggests they go outside for some fresh air.
During "The More You Moe, The Moe You Know (Part I)," BMO tries to consult with Air about his decision for growing up.
As the name suggests Bubble's first appearance was that of a bubble, a liquid sphere enclosing air or gas. It has two dots for eyes, a mouth, and a tongue. It was also capable of blushing. After being popped by Jake, Bubble's insides became one with the air, an invisible gaseous substance, but it could still communicate with BMO.
When Bubble was first introduced it asked if it could wallow with BMO who appeared to have given up after tumbling down a hill. Bubble stated it thought about giving up on things and even accepted getting potentially eaten by a carnivore. But when BMO perked up, Bubble's mood shifted to match and it journeyed with BMO to find their home. Gradually it began to become fearful of the forest and at one point hid behind BMO for protection against the carnivores it claimed existed. When they found a baby and BMO asked about the baby's name, Bubble said that his name should be Sparkle, but BMO overrode the suggestion by calling the baby Ricky, causing Bubble to be visibly disappointed. Bubble showed selflessness when BMO and the baby were in danger of going over a waterfall; Bubble didn't hesitate to attempt a rescue. Also, it was more concerned about the baby getting back to his mother, and told BMO to "let go."
When BMO was feeling down Bubble tried to cheer them up by telling them about a clear path to BMO's home. Upon arriving at the Tree Fort, Bubble felt the urge to confess his feelings about BMO, and proposed, even though it didn't understand what marriage meant. When BMO accepted, Bubble was ecstatic and laughed happily. Now as "Air," Bubble still wants to be around BMO forever.
I'm a bubble.
No, see, that's what's so great. Now we can be together forever, BMO. Every minute of every day. No more privacy, no more quiet, no more alone. Every room you ever go in, I'll already be there, waiting, forever and ever, until the end of time.