Yeah, gotta feeling it's the ubiquitous brother or cousin. It's a romcom, by law* there has to be some form of tension for both sides of the relationship.
*yes, yes, I know there isn't an actual law, but it sure is the meta.
Obviously Suzuki is her brother. Seen that before many times.
And why summer was skipped? Why we didn't see beach time? I love these characters, I love their designs, but also it would be cool to see them in non-school enviorement having fun without much worries. Meh.