I need a word to describe something like this:
Disclaimer: I'm not a Smallville fan.
A half-smile or a sappy smile.
"Smirk"? ;) It's often used in a sarcastic way, but not always.
But I guess there is no other word for "half-smile" in English. Especially when used in romantic situations, you need to use your imagination instead and use a metaphor. " :D
Collins is happy with half-smile
A related term is sly smile
More often than not, such a smile has a more knowing look to the eyes and is a bit less innocent in appearance.
Another term is wry smile
While often subtle in appearance, such a smile is often described as slightly twisted
Yeah, I've squeezed my mind and scoured the internet, but I don't think you'll find a word in English for a smile like that. The closest you can get is a combination of words, like in Mari-Lou's list.
Other languages do have words for various degrees of smiles - beyond smirks and grins - but not English, apparently.
Presumably you're looking for positive connotations, and linked to romanticism.
A timid / tender smile
A warm / sympathetic / fond smile
An amiable / engaging / thoughtful smile
A knowing / charming / suggestive smile
A quasi smile.