Someone who's ungenial isn't particularly pleasant or cheery. An ungenial greeting from your new piano teacher doesn't bode well — you might want to find a friendlier one!
When it describes a person, ungenial brings to mind someone who's cold, aloof, and unsmiling. You can also use it for situations that aren't particularly welcoming: "The distinctly ungenial winter weather didn't make me eager to get out of bed in the morning." Genial is now defined as "kind and pleasant," but it originally meant "pertaining to marriage," from a Latin word meaning "pertaining to marriage rites." Even today, you wouldn't want to marry someone who's ungenial!