Air

Bubble (later referred to as Air) is a lost bubble who appeared in "BMO Lost". Once BMO found itself lost in a forest Bubble brought itself up to help BMO back to the Tree Fort. Bubble first met BMO when it tumbled down a hill and cracked its face plate, Bubble emerged from a cracked mug and said to be lost in the forest for a long time and feeling the need to give up. However, BMO's determination to get home kept up Bubble's mood. After some trials and spending so much time together, Bubble and BMO developed a friendship, making it feel no longer lost and instead more at home with BMO.

Appearance
As the name suggest Bubble's first appearance was that of a bubble, a liquid sphere enclosing air or gas. It had 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.

Personality
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 given 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 its 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, BMO override the suggestion by calling the baby Ricky, causing Bubble to be visibly disappointed. It shows selflessness when BMO and the baby were in danger of going over a waterfall, Bubble didn't hesitate to attempt a rescue. Also being more concerned about the baby getting back to his mother by telling BMO to 'let go'.

When BMO was feeling down Bubble tried to cheer it up by telling it about a clear path to BMO's home. Upon arriving at the Tree Fort, Bubble felt the urge to propose to BMO, 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.

Quotes
"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."