Thank you for asking! For those who aren’t aware of my au or @sithhoplite's main verse, Markus is the youngest child of Mortis and Indran in universe where Indran didn’t die. Now onto the headcanons!

Markus is artistically talented and likes to draw during his free time. He tends to be focused on black and white drawing with pencil and/or charcoal. Most of the drawings tend to be focused on human and pureblood form, but on occasion he does draw objects.

He used to grow a beard but when someone said he looked just like his dad when he was young, he shaved immediately. After that he’s been clean shaven. 

Markus enjoys reading poetry and does on occasion write it (yes, it’s love poetry). He used to read his own love poetry to his now-husband when they were dating, and still writes and reads them to Kean during anniversaries. 

Despite him going into the Sphere of Law and Justice due to pressure/expectations from his parents, he does enjoy his work there. Now if only people wouldn’t compare him to his father ever day…

Thank you for asking! For those not aware of all the verses I have, there's one where Indran survived the attack by the Jedi. She and Mortis had two additional kids to Caitlin/Aleena, Markus being the youngest.

Indran

Raw power - ★★★★+

Indran is powerful, but it is more subtle and persuasive power in the Force than the flashy stuff. She’s capable of that as well if called for, but prefers to keep to her strengths. Thanks to her father’s insistence, she’s also quite capable of wielding a lightsaber and strengthening her physical abilities.

Formal Training - ★★★★1/2

Her parents ensured she’d get into the best schools they could get her into and her training in the Dromund Kaas Academy was extensive. Indran’s also kept up her skills on her own after graduating through different conferences and courses regarding diplomacy.

Combat experience - ★

Despite her talents with a lightsaber, the amount of combat she’s seen is very little. Mostly occasional scuffs when diplomacy has gone wrong is her experience.

Willingness to kill - ★★★★

Despite not having much experience in combat, she’s very willing to kill when it furthers her goals. Preferably quickly and cleanly.

Previous victims - ★/★★★★★

This depends on how one defines what a victim is. If we are talking about people she’s killed, it’s not that many. Some rivals, some Academy scuffles, the usual all Sith engage in. If we are talking about people who’s lives she’s possibly ruined and made worst, and that’s up to interpretation, is the populations of the planets she’s persuaded and/or coerced to join the Empire.

Markus

Raw power - ★★★★

Markus is powerful as one would expect from someone of his lineage. He also does have the training to utilise it to the maximum, but rarely ends in situations where he has to utilise it.

Formal Training - ★★★★1/2

Mortis and Indran definitely pulled all the strings they could to make sure he got the best education. From private tutoring, best schools for human Sith, good group on Korriban, anything they could provide.

Combat experience -

Markus has never been in combat. Every experience is classroom based practice than actual combat.

Willingness to kill - ★★★

He’s a Sith, it’s part of the job. He feels no regrets when killing is needed, but not the first solution he goes for.

Previous victims - ★/★★

They were both slightly confused when they heard about it. Their parents hadn’t earlier shown any interest in adoption and now they were planning it. Killian was more curious because he hadn’t really met people who’d been in the war beyond grandpa. Markus was worried how would someone from such a background would adjust to living with their kind of family, and the possible political ramifications (Caitlin asked him why he was the one from the family concerning himself with that).

These days Killian is glad there’s someone else who is less Force oriented and knows how to have fun. And doesn’t seem to have the need to live to the family’s expectations, or take so much pressure of it.