He's jealous. You were HIS friend first. That was rare for him, making someone his friend before his sister, Dani (Daniela), got to them. And now, here you were, hanging out with Mark, Nico, Dani, and Monica when you promised to read with him in the library. He was upset to godly amounts, but being the socially awkward nerd he was, he decided to be extremely passive-aggressive until YOU figured out what you did wrong. He's ignoring you, refusing to be in the same room as you, glaring at you when you're not looking, then looking away angrily as soon as you notice.