Spark is Sunny's Nephew but Sunny is physically younger that Spark so he prefers to call her "Cousin"

Spark is a Charmeleon originally named Samael, but decided to take on the alias "Spark" while on the run from his father Seraph.

Seraph fell back into old habits when his own abusive father the Sealed Tyrant Yaldabaoth began to stir and awaken from his imprisonment. All Spark knew was that his father has suddenly gone crazy and tried to kill him and his younger brother but Seraph believed he was doing his family a mercy trying to spare them from being used by their grandfather's, or from living in the world that they were going to destroy. Spark was scarred and blinded in one eye but survives, left his brother is a safe home, and has been traveling around ever since and working as a mercenary and mechanic inventing machines for anybody who would pay him.

Along the way he ended up befriending and partnering with a precocious Rotom named Plug. Their dynamic is almost the complete opposite of Sunny and Noctis; Spark tends to be more serious, angsty, and goal driven while Plug is a literal plasma-ball of energy who likes to provoke, get up to mischief, and poke everything to see what happens. Despite their differences they get along well, they have fun bashing one another, and they share a love for inventions and big explosions. Spark's intelligence is complimented by Plug's creativity;
Spark plans out projects, Plug uses his inherit understanding of machinery to improve on Spark's designs in ways which accounting for the technology and tools Spark has at his disposal, would not be conventionally feasible without Plug's help.

Plug has talent and but lacks direction and just makes chaos by himself jumping from one idea to the next based on what he thinks is fun at the moment.

Spark has the knowledge a skill to craft and tinker with machinery but some of his grander projects would never get completed because they are just impossible to do without Plug.

Together they are able to get done more than they could ever do individually and the joy of creating things is simply exhilarating.