Board Thread:Questions and Answers/@comment-26005711-20151226104253/@comment-29289594-20170405162329

Hisoka is boastful about his powers.

Gon flat out managed to land a hit on him in Heaven's Arena. Hisoka only held back the strength in his attacks - he did not hold back while defending - there's no point in that.

Heaven's arena Gon would never land a hit on Killua, so he probably wouldn't do it to Illumi, either.

Overall, there have been hints that Illumi is far more twisted than Hisoka.

"I've got issues, but you're just as bad."-Hisoka to Illumi.

On top of that, we have Illumi's maniacal laughter. At the same time, the strength of his aura made his hair float around, something we've only seen before with adult Gon.

Hisoka is also known for underestimating his opponents, given how much he underestimated Chrollo.