Leandros speaks with little concern of others. To him, all action must be in service to the Emperor and those who do not give their all in service must have their motives questioned. Since becoming an Ultramarine, he no longer remembers how it is to face the world as a human. Throughout the conflict on Graia, he is highly patronizing to the surviving Imperial Guard soldiers.
Being new to the fold, Leandros is under the tutelage of Chapter Sergeant Sidonus. He also admires Capt. Titus because he believes Titus is the example of how an Ultramarine should be. However, being young (in relation to the average Ultramarine), he seeks to prove himself and his abilities by overly pursuing what he believes is the noble course of action.
Leandros is with Titus when he decides the most effective way to attack the enemy above Graia is with assault jump packs. Leandros protests that this plan of action is against those outlined in the Codex Astartes, but the concern is quickly dismissed by Titus' challenge of keeping up.
Upon reaching the surface, Leandros is already with his mentor Sergeant Sidonus when Titus links up with them. Leandros accompanies Titus and Sidonus as they destroy the Ork captured gun and proceed to the Maufactorum, where Inquistor Drogan has stored very valuable and deadly properties and a Titan. After the destruction of the gun, it is realized that Titus has an unknown ability to resist the dark powers of Nemeroth. Leandros begins to voice concerns of the ability, but was silenced by Sidonus. Later in their conquest, Leandros becomes distraught by the death of his mentor. Titus urges him to turn his sadness into rage to be directed at Chaos.
After the last mission, Leandros is with Titus and Lt. Mira as members of the Black Templar Space Marine Chapter and Inquisitor Thrax arrive planet-side. It is discovered that Leandros has charged Capt. Titus with "being corrupted by Chaos', that no "true" Space Marine should be able to interact with a link to the Warp the way he had. Agreeing to submit himself to inquiry by the Inquisitior, to save those under his command, Titus has a final conversation with Leandros. He states " The Codex is a set of rules by which Marines should guide themselves by, but it is the manner in which you conduct yourself, within those guidelines, that truly makes a Space Marine, and you have failed." The last seen of Leandros is as Titus boards the ship, he turns and walks away in apparent shame.