- "I belive there are only two kinds of women: The ones who look good in the rain and the ones who don't."
- ―Leonard presenting himself to Chidori.
A child prodigy and Whispered like his younger twin sister, Teletha, he is known as "Mr. Silver" in the executive circle of the Amalgam organization. However, Leonard surpasses his sister in both knowledge and physical prowess, even defeating the ARX-7 Arbalest piloted by Sousuke.[21 He also claims to be in love with Kaname Chidori. In the anime version, Full Metal Panic! The Second Raid, he earns her contempt by forcefully kissing her. In The Second Raid, he is chaperoned by two miniature ASs. It is speculated that the whispered knowledge inside of him has allowed the creation of the Codarl and Codarl-m (Mithril codename Venom) type AS types (as Bunny Murata was to the Arbalest), the miniature Alastor types and the Leviathan-type underwater fighter/AS hybrids. In the novel, Continuing On My Own, he is seen piloting the near invincible Belial type AS, and shows the strength of the machine and himself as a pilot by completely obliterating the Arbalest (Sousuke managed to survive with minor injuries). He was also the one who kidnapped Kaname & is currently being pursued by Sousuke, the remaining Mithril remnants & also by the anti-Leonard faction of Amalgam for his betrayal towards them. Exactly what are the actions that he did towards his own organization has not yet been revealed as the story is still continuing.
Admiral Borda is familiar with him, aside from Tessa and their parents. However, Admiral Board doesn't know about Leonard's ties to Amalgam.
Leonard Testarossa is Teletha Tesstarossa's older brother and a high-ranking member of Amalgam. He is the series' primary antagonist and an archnemesis to both his sister and Sousuke. His personal AS is the Belial.