Tesseract

Tesseract was a powerful Russian assassin who appears in the fifth book. His main objective during Mortal Coil was to assassinate Davina Marr. Tesseract was always seen wearing a metal mask, though he has several and the designs do vary. Without it, he would die due to a rotting disease that plagues him as his mask injects a fluid into his face to keep it at bay. This also granted him great resistance to most toxins.

Although much is unknown about Tesseract's personality, it is shown at the end of Mortal Coil that he had no regrets in life and he did what he did because it was life. He appears as a stoic and powerful mage with a mysterious background, but in his dying stages he states that he will miss his cat, and his dying wish was to see and feel the sun on his face without the mask which keeps him alive. It is evident from this that Tesseract isn't a completely heartless killer, as he values sentimental things. He appears to be a deep thinker and acknowledges the fact that he makes mistakes. He also has a cat which is just called "Cat" he seems to have great fondness for it, saying he will miss her when he is dead.

Tesseract is an adept mage with a rare ability called Bonebreaker. It is also evident that Tessearct used his magic effectively and could take down even groups of extremely proficient mages including; Tanith Low, Valkyrie Cain, Skulduggery Pleasant, Ghastly Bespoke and Erskine Ravel.

While he is not described as possessing superhuman strength as part of his magic, he is an enormous man with a massive build and a large amount of muscle mass, granting him enough strength to overwhelm any opponent without superhuman strength. He is also shown to be incredibly stealthy, sneaking up on both Tanith Low and Davina Marr without making a sound, feats made more impressive by the fact that both opponents were in fortified locations at the time. He displayed a high level of martial arts and combat skill during his murder of Davina Marr, countering or defeating all attempts to capture him despite being heavily outnumbered. Finally, he is shown to possess a high tolerance for pain, fear and stress, as demonstrated by his refusal to be cowed by being buried alive and his stoic attempts at completing his goals at any one time, no matter how he is injured or what happens to him.