Why does this sentence need "yourself"?

What does it mean by "haul ass"? Is that a slang?

Yes, it is crude slang! It means to be fast or move quickly. Ex: We need to haul ass out of here before we get caught. Ex: It's the police! Haul ass!

Is "comfy" an abbreviation of "comfortable"? Is this a general expression?

Yes, "comfy" is a casual abbreviation of "comfortable". It is a general expression and commonly used. Ex: My bed has lots of pillows and blankets, so it is very comfy.

What does "I so am." mean? Is it a common expression?

"So" in this context is used to mean "really". So "I so am" means "I really am". "I so am" is not a general expression, but the use of "so" in this way is common. However "so" is a lot more informal than saying "really", and is not as common to use in this way. Ex: A: He is handsome. B: He so is.

What does "even" mean here?

In this video, "even" is used to emphasize her statements. In the similar context, "even" is commonly used to emphasize anger, severity, or surprise. Ex: Don't even think about it. Ex: He has never even heard of Jennifer Anniston! Ex: You don't even have a chance of winning the lottery.

Why does this sentence need "yourself"?

Great question! The sentence here does not need to include "yourself", however, adding it emphasizes his singularity and implies that the Pokemon partner is possessive, and it is only for him. It is commonly used in American English. This grammar structure can be used for other pronouns as well. Ex: She just brought herself a new puppy. Ex: After I walked downstairs, I found myself forgetting what I was looking for.