Learn Spanish (Latin America): Ella no viaja sin mí.

Mí means "me" when it comes directly after a preposition, like sin (without), para (for), or de (of/from). You cannot use the subject pronoun yo (I) or the object pronoun me (which is used directly with verbs) after a preposition.

Why does mí have an accent mark?

The accent mark shows the difference between mí (the pronoun "me") and mi (the possessive word "my"). For example, sin mí means "without me," but mi viaje means "my trip."

Does no always go right before the verb like in no viaja?

Yes! In Spanish, to make a sentence negative, you simply put no immediately before the conjugated verb. So "she travels" is ella viaja, and "she does not travel" is ella no viaja. You do not need a helper word like "does" in Spanish.

Can I drop the word ella and just say no viaja sin mí?