Crona

For those who haven't watched Soul Eater, you should. It's a fantastic introductory anime with a great cast of characters. It also introduces maybe the most emo cartoon teenager to date in the form of Crona. If you ever shopped at Hot Topic, you'd totally be friends with Crona.

Jake the Dog

Although a wide variety of Adventure Time characters would make for great friends (the Earl of Lemongrab and Lumpy Space Princess would also be on our short list), there's no doubt that Jake would be the best option. He's as loyal as they come and can shapeshift into whatever you need him to be at any given time.