Both Uses of

mischievous
in
The Lost Hero

Leo looked like a Latino Santa's elf, with curly black hair, pointy ears, a cheerful, babyish face, and a mischievous smile that told you right away this guy should not be trusted around matches or sharp objects.†
Chpt 1

She stood next to a sandy-haired guy about fourteen or fifteen, with a mischievous smile and ragged leather armor over a T-shirt.†
Chpt 14 *

Definitions:

(1) (mischievous)
playfully causing minor trouble; or describing the smile of someone doing so

Much less commonly, mischievous can reference real harm without any sense of fun. But in modern writing, that usage has largely shifted to other words like malicious, destructive, or damaging.
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

In law, mischievous references a property crime such as vandalism or graffiti. Very rarely, the word can reference someone or something causing serious damage.

In archaic literature mischievous often refers to bad behavior without any connotation of playfulness or of the harm being minor.