Uh why are you taking such offense to me saying that she looks young? I thought you guys had no problem to admit that you're into young girls.
Look, as a character I like Tessa. Not as much as Chidori but I do. She especially shows maturity in TSR.
But the whole point of her character design is to capitalize on moe loving otaku and to contrast her position with her appearance. In other words, she looks really young and sometimes she acts like it but she's also the captain of an entire submarine and military operation.
It's like how kids pilot mecha but it's even more jarring because everybody around Tessa is older. Why do you think her second in command is a grizzled old man?