Teletha "Tessa" Testarossa is the clumsy captain and creator of the Tuatha De Danaan submarine and Dana, the sub's Artificial Intelligence. She belongs to Mithril, an Anti-Terrorist private Military organization. She is a U.S. American by birth but has lived on submarines and military bases most of her life. She was born December 24, 1984 and is a child prodigy who achieved the rank Colonel by age of 16. She has silver/platinum hair and gray eyes. Later in the series, Tessa is revealed to be a Whispered. Her high level of mathematical and scientific prowess is dwarfed by her older brother's, which gives her a sense of inferiority. Though not very athletic, she is shown to be a very good swimmer in episode 8 of Full Metal Panic? Fumoffu. Like Sousuke, she hates alcohol, saying that it kills brain cells. She also has an interest for Sousuke extending beyond a professional one, generating a love triangle she shares with Sousuke Sagara and Kaname Chidori. Even though Tessa and Kaname compete against each other for Sousuke, they are good friends in reality. During many episodes they talk about Sousuke and his unique military charm. Tessa is very open in her feelings to Sousuke, something that causes arguments between her and Kaname often. This is seen several times during the anime, such as in Episode 13, where Sousuke helps her to train and get ready for a duel with Melissa Mao, Tessa accidentally falls off the boarding ladder of the M9 she attempts to pilot and lands on Sousuke. When she realizes that she is on top of him, she apologizes, but also says that she feels lucky. When Sousuke tries to slip out from under her hold, she refuses to let go and holds on tightly. She also accidentally sent Kaname into a jealous rage when she pretended to be Sosuke's girlfriend after Sousuke and Kaname came to his apartment as Tessa was stepping out of the shower. Tessa shows her affection for Sousuke in Full Metal Panic? Fumoffu! when she attempts to sneak into Sousuke's bed during her vacation, but Mao handcuffed her to herself. Unfortunately she is just a little too open in her affections which causes Sousuke some mental stress (shown in Full Metal Panic? Fumoffu!), when Sousuke faints as he is worried that he will be alone with Tessa (as she came to visit his school during that time on vacation) and that something may happen between them (mainly because Commander Mardukas threatened him if anything were to happen between Sousuke and Tessa while she stayed at Sousuke's residence; threats include throwing Sousuke into the TDD-1's reactor until he starts to glow). She was elated at the end of Full Metal Panic! The Second Raid when he finally called her Tessa.
In the novel, A Dancing Very Merry Christmas, which took place chronologically after the events of The Second Raid, she professed her affections for Sousuke, only to be devastated when the latter admitted that he was more fond of Kaname Chidori.
Tessa has some interesting qualities also. Being Captain of the Tuatha De Danaan, she is a very capable strategist. She displayed some very impressive military strategical skills at several points, such as the first two episodes of Full Metal Panic! The Second Raid, where she helps Sergeant Sagara escape death and also help their team get out of a tight situation in an underground operation. Also, during the last two episodes of Full Metal Panic! (first season), she helps prevent the Tuatha De Danaan from sinking, and performs some very skillful actions to save them from destruction, not once, but twice from torpedo attacks. She also has a habit of playing with her ponytail.
Sometimes however, as stated by Kurz Weber, "whenever Tessa was told by someone she couldn't do something, she made it a point to prove them wrong." (this leads to an Arm Slaves match involving paintball bullets between Tessa and Melissa Mao over the argument as whether Tessa can pilot an Arm Slave or not). Tessa seems to have some trouble tolerating her job at times due to the position she has. Some scenes have shown her irritated by what she does while other scenes show her slightly depressed by it. She mourns any death greatly and strives to reduce casualties in any mission. Nevertheless, she seems to have a large amount of stamina, and is able to cope with the stress of her tasks.
She treats everyone in the TDD-1 like her own family and cares about them to the point that she refuses any order for her to step down as TDD-1's commanding officer, such as the time when Admiral Borda suggests that Tessa returns back to R&D instead.
Teletha Testarossa is an American by birth, as she was born in the USA's northeast (it is intimated that her cultural background is akin to Boston Brahmin), to a family of Swiss and Austrian descent, as revealed in the short story 猫と仔猫のR&R. It can be inferred that she is of Italian descent judging by her last name Testarossa. She traveled the world extensively as a child; however, she has spent a great part of her life living on submarines and Iams.
When she was younger, she lived in Okinawa for a while, while her father suggested she attend a Japanese school there, but as she did not mix very well with the other Japanese students, she was transferred to another school, this time on her father's military base.
Tessa is a Whispered, and thus is gifted in science and mathematics. When she was six years old, she had derived the answer to one of Albert Einstein's scientific theories. On a side note, her twin brother, Leonard Testarossa, did this at age four, giving her an inferiority complex, as she states in Episode 10.
She worked her way up very quickly in Mithril, and became Captain of the Tuatha de Danaan after being commissioned by MITHRIL. It is also speculated the knowledge found within her may have led to the construction of TDD-1, due to the fact that Gauron refers to the existence of a Whispered knowledgeable in the field of submarine construction. She also mentions having had a hand in the construction of its AI, Dana.
When she participated in the "hijacking" of the Pacific Chrysalis in "Dancing Very Merry Christmas", she used the alias Teletha Mantissa.