Formal:
* Moi aussi je te manque. (Literally: "Me too, I miss you.")
* Je te manque aussi. (Literally: "I miss you too.")
Informal:
* Moi aussi ! (Literally: "Me too!") This is the most common and casual way to say it.
* Je t'aime bien aussi. (Literally: "I like you too.") This is a softer way to say it, implying that you miss the person but are not necessarily romantically interested.
More specific:
* Je t'ai manqué aussi ! (Literally: "I missed you too!") This emphasizes that you missed the person while they were away.
It's important to choose the appropriate expression based on your relationship with the person you're talking to.