Ketchup

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

Canon
Unfortunately, this couple has never met in canon. This is due to them being from separate series. These series also currently lack a crossover episode. Despite this, these characters share an abundance of similarities. These canon similarities cover both physical appearance, personality, and interactions with other characters.

In terms of appearance, both have a similar hairstyle. Kai's hair is brown. Hiccup's hair is auburn. However, both sport a messy, slightly dishevelled hairstyle, which makes them appear similar. Additionally, both seem partial to the colour red, and add this to their wardrobe and stylings to differing degrees. Both characters also sport a flame sword at different points in their respective franchises, demonstrating that 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. However, 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.

There are various reasons why Ketchup is shipped, depending on the fan. Their personalities mesh well - both are reckless, with a thirst for adventure, and of course, a soft spot for dragons. Their skill sets also make for an interesting dynamic; while both have been blacksmithing since a young age, Kai is quite horrible at it, while Hiccup is extremely proficient. Some fans like to think Hiccup would be a good mentor to Kai in smithing, and that they would bond over the activity and long hours over the forge, eventually becoming very close and falling into a relationship.

The two also share a similar family life, in regards to their parents. Both characters lost their fathers at a young age, and both of them were deeply affected by this. They would probably be able to bond over this, if they ever were to meet, and share their experiences with how they've coped. Given that both have taken on a respective "hero" role, they would also likely be able to share their experiences with that, and how this lifestyle may have been a burden to their well-being at times. In fanon, these two are a great comfort to each other.

Finally, fanon acknowledges how the two might see themselves in each other. For instance, Kai appreciates Hiccups green eyes because they remind him of his old hopes and grudges for not being picked for green ninja; it reminds him how hes grown and changed. They can both see the same stubbornness and impulsivity within each other, and perhaps in some cases this allows the two of them to learn and grow together as they navigate their lives.

Unofficial
A few PMVs can be found featuring both Hiccup and Kai, and while there aren't very many yet, fans may associate the pairing with the band Maroon 5.

Variations
Ketchup is not exclusively romantic. Many variations portray Hiccup and Kai as merely close friends, and sometimes even as family. This is commonly found in crossover fanfiction between the two franchises.

YOUTUBE

 * Kai and Hiccup Playlist

Platonic

 * How to Train Your Dragon/Ninjago on FanFiction.net

Familial

 * The Viking Chief's twins by Lava_Serpant on Wattpad