What does Spark Elf's passive do?

The bonus applies to the whole team rather than just Spark Elf, so it pays off most in a team built around fire damage. Put Spark Elf beside a fire attacker such as Infergon, which can be the main damage dealer against bosses, and the passive boosts the hits that matter most.

The passive is only one of Spark Elf's fire buffs. Its core skill, Inspiration, raises the team's fire damage by 16%, and its ultimate gives the team a buff that boosts Fire-type members when it lands. These buffs stack, so the team's fire damage bonus can reach 70%. That ceiling is why Spark Elf counts as a support for a fire team, not as a damage dealer of its own.

Not settled: The game does not say which buffs, or how many uses of each, it takes to reach the 70% fire damage cap.