At first, their relationship might become strained. Zane's anger could make Syrus confused and a bit scared. Syrus might try to avoid Zane, not knowing how to deal with his anger.
One possible reason could be jealousy. Maybe Syrus is getting more attention in the fanfiction world, and Zane, being a proud character, doesn't like that. For example, if Syrus is praised for his skills while Zane feels overlooked, it can make him mad.
Maybe Zane and Syrus get trapped in a strange virtual world created by some glitch in the digital system. They have to rely on each other's skills to find a way out. Zane's logical thinking and Syrus's knowledge of the digital realm might be combined to overcome the various obstacles like malfunctioning programs and security barriers.
It could be his rival. Rivals in Yu - Gi - Oh are always trying to one - up each other. If his rival manages to steal a rare card that Zane has been eyeing, or if the rival taunts Zane about a recent defeat, that would surely make Zane mad.
It's quite normal. In a fandom, people are interested in different aspects of the characters. Zane might be curious about Syrus's backstory, relationships, or potential storylines that fanfiction writers have created. It's just another way of engaging with the fictional world they are part of.
Another aspect of their brotherly love in fanfiction could be the way they support each other during tough times. Syrus might be having a hard time with his self - confidence, and Zane would be there to encourage him. He could tell Syrus about his own insecurities when he was younger and how he overcame them, thus inspiring Syrus to grow and believe in himself more.
There's no way to really say if it's common without knowing more about Zane. If he has a lot of free time and is a die - hard fan of the source material involving Syrus, then it might be a regular pastime for him. However, if he's not that into the whole fan culture or Syrus specifically, it would be less common.