This is Ichimaru Gin's near-permanent expression.Shinji Hirako has an even more classic version of a Cheshire cat grin.Kisuke Urahara, particularly before unleashing one of his training regimes on someone.And in one of the OVAs? Grelle literally had the Cheshire's smile while playing the Cheshire Cat!.It probably means someone is about to get hurt. For some these cross the boundary into Glasgow Grin because of the fact they're missing bits of skin. Lots of the eponymous Titans of Attack on Titan sport unnerving grins, even when they logically should be in pain.Or, for that matter, how their face manages to do that. If you ever see someone with the Cheshire Cat Grin in real life, vacate the premises before you can discover what they're smiling about. If this smile is a permanent feature, it's a subtrope of Frozen Face if it's cut into the character's face, it's a Glasgow Grin. Common trait for The Gadfly, and serves as a warning that they're going to pull their antics on someone. If it's an anime, expect a Cat Smile instead. Heroes and heroines who are slightly eccentric - or slightly deranged - favour the Cheshire Cat Grin. Strangely, it's usually done by heroes who have a Zany Scheme in mind, rather than by villains, who tend to prefer the Psychotic Smirk or the Slasher Smile a rough villainous equivalent would be to start laughing ominously, which is not a good thing for the hero to hear when cornering them. It occasionally has sexual connotations, particularly when there's a predatory quality about the grin.
For example, the mouth may be a broad grin, but the eyes definitely have a wicked gleam in them. Generally, this is drawn as a smile with far too many teeth on display, where the mouth doesn't match up with the eyes, going practically from ear to ear.