Thaddeus is the leader of a three-man Assault Squad the Blood Ravens Chapter. He and his squad arrive in the mission "True Enemy" to help slay the xenos enemies. He and his squad arrive just in time to defeat an ork ambush on an impossible to get to ridge. He is very useful defeating an Eldar Warlock who is very susceptible to melee attacks.
He was an undercity gang leader on Meridian until Davian Thule recruited him to the Blood Ravens. He showed great prowess in hand-to-hand fighting having a lot of practise on Meridian. He may no longer be a gang leader but he still has the bravery and strength. Unfortunately he also retained the cockiness.
He can be armed with many close combat nasties from the humble chainsword to the tank destroying powerfist. This includes power swords and axes. He can also equip bolt pistols, plasma pistols and bolters. He can also be outfitted with Terminator armor if he has the Terminator Honors trait. This allows him to equip a thunder hammer/storm shield combo. If you upgrade his ranged damage traits he can be armed with flamers and plasma guns.
He does high melee damage in the beginning but low health and energy. This can be a problem because up close you take more damage than at range and you need energy to assault jump all of the time. Focusing on Health, Strength, and Will traits is what suits Thaddeus best. In addition, acquiring the Terminator Honors traits and equipping him with a good Terminator armor will greatly increase his survivability, but he loses access to several traits that affects Assault Jump, rendering many of his traits useless when in terminator armor. In the game the best tactic to use is to surprise some deadly ranged xenos with an assault jump before they can do any damage.