Ketchup

Ketchup is the slash ship between Hiccup and Kai from the How to Train Your Dragon and Ninjago fandoms.

Canon
These two have never met in canon, as they are from two separate series, although there are several aspects that make them similar.

Appearance-wise, both have similar hairstyles. Although Kai's is brown and Hiccup's is auburn, they both sport a messy, slightly dishevelled hairstyle, which makes them appear similar. As well, both seem partial to the colour red, adding it to their wardrobe and stylings (to different degrees).

Both characters also sport a flame sword at different points in their respective franchises, meaning they both have an appreciation for fire and the power it wields, as well as a knack for controlling it and bending it to their own will.

Finally, both characters have a relationship with dragons. Each of them has ridden and bonded with a dragon, although Hiccup's relationship with his companion lasted much longer than that of Kai's. Both seem to share a deep connection with the creatures, even sharing a similar scene in which the respective character reaches out to caress a dragon on the snout. Both characters also ended their relationships with their respective dragons, although Kai's seems to have been more permanent, as his dragon was hunted for sport, whereas Toothless remains alive for the time being.

Fanon
While Ketchup isn't the most popular ship, it certainly has the potential for more. It's not difficult to find crossovers between the two series, and a bit of content for the ship does exist if it's sought out.