The phrase "fits in" typically refers to someone or something being suitable or compatible with a certain group or environment. Other synonymous phrases one could use include "blends in," "meshes well," "conforms to," "matches with," "integrates smoothly," "assimilates easily," "adapts," "belongs," "suits perfectly," "harmonizes," and "gets on with." Each of these phrases emphasizes the idea of being well-suited or compatible with a certain situation or group, and can be used in a variety of contexts, from describing a new employee who fits in seamlessly with their colleagues to a house that blends in well with its surroundings.