With very few units let's crack into it
Medusan immortals: these guys embody "hold the line" 3+ 5++/6++ and no enemy charge bonus due to shields -1str t4. These guys are a massive pain to get rid of. Can be give Volkilite charges as an upgrade which effects their next rule: they can only snap shoot instead of making sweeping attacks.
Gorgan terminators: t4 2+ 5++ -1str 4+ blind, lighting claws (upgrade) i3. Expensive but effective. These guys are terminators that hold front lines. With them causing blind getting up close and charging is great because any shots that you pass with your 2+ save for over watch will blind the opponent, letting your other wise derpy I3 go first.
Castrmen orth: 80pts 30k cronos but better. Can go in any tank you purchase. Has tank hunter and gives your army +1 vehicle damage when ramming. Also he can get out but only has 3+ 4++ so not suggested. Can be in a spartan, super heavy etc, anyone else thinking fell blade.
Autek Mor: iron father praetor with cataphractii terminator, which means slow and purposeful, no over watch. He does get +1 str & toughness (he does have -1 intitive and -1 attack) servo arm (which gives him +1 attack lol) paragon blade cause they're fun (+2 Str so he's Str 7 with most of his attacks and ap2). Also has a Volkite charger. Fearless which is nice, preferred enemy infantry so rerolling the ones that he would fail to wound on. Servo arm is Str8 i1 and on the charge he has 4 str7 ap2 (6s= ID) And because of Involite armour he can't be ID from shooting 2x toughness.
And that's actually all of the special characters and units for the robot space marines.
'I have seen my Lord Ferrus handle molten iron without flinching, bending it to his will with barely a concern that such a deed is beyond mortal men. Have no doubt that by those same hands will revenge be exacted upon the traitors, have no doubt that the stain on our honour will be drowned in blood, just as the ruby-hot blade is quenched by the fire.'
— Ironwrought Orzhar, Avernii Clan-Company, on the eve of the Dropsite Massacre (Horus Heresy Book 2: Massacre, page 231)
Again thanks to Warhammer 40k quotes Facebook page.