I started playing the game after Caleb came in as a LI.
But the first part of the MAIN STORY. You know, Under Deepspace, was what the game launched with and the only LI's available in it were Xavier, Zayne, and Rafayel.
Caleb was JUST an NPC. Like Tara. Grandma Josephine. Etc.
And Sylus could have easily have just been an NPC as well. As throughout Under Deepspace ALL 3 of the OG3 parts make mention of him and / or Onychinus.
Xavier (and Jenna): flat out name drop both Onychinus and Sylus when talking about modified Protocores.
Zayne: Doctor Noah makes mention of Onychinus when MC confronts him (with Zayne) to give her answers of the little info he knows about the Aether Core.
Rafayel: helps MC navigate how The Nest works so that she can be taken into the N109 Zone all so that she can go after Sylus as she believed HE (and Onychinus) was responsible for Josephine's (and Caleb's) death(s).
Infold, could have legitimately just kept the game as the LAUNCH THREE love interests and just had Sylus and Caleb (when he eventually returns to the plot) as normal NPCs that MC needs to confront / deal with emotionally.
They wanted to include them to the LI roster as well.
And the same can be said for Valko.
Because we know from the live stream that Valko's MS chapters were supposed to answer questions about the AETHER CORE!!!! You know, the very thing that MC has been questioning since she received the notes from Zayne that Josephine gave him to give to her?!
Valko could have just been brought in as an NPC. But just like with Sylus and Caleb, Infold saw potential in making him a full LI.
EDIT: The game launched with the OG3 (Xavier, Zayne, and Rafayel) and basically 2 (Sylus and Caleb) that were part of the NPC cast.
Sylus = The first in a FREE UPDATE.
Caleb = The second in a FREE UPDATE.
Valko = would be the third in a FREE UPDATE.
But all three POST LAUNCH LI's are effectively (taking the typical gacha mechanics out) FREE DLC characters for the game.