Since Sterling has been tasked with following Sadie around by his sponsor, Sterling had a lot of time to obverse Sadie, and ultimately fall for her. Regardless of route, Sterling views Sadie in a romantic light, though one must follow his main adventure for him to actually confess. He is a bit of a wild card as due to his mission, he has to keep secrets from Sadie and sometimes has to harm her for her own good (or for his sponsor's own good). Due to this, his and Sadie's relationship, romantic or not, is the most strained of all the romance options in 1931. Even so, whenever they have to explore together, the two work well together and are able to put aside their differences in goals: Sadie an archaeological discovery, Sterling something to appease his sponsor. Sadie is aware that much of what Sterling is doing is not really of his own free will, his sponsor ordering him or his father ordering him, so she is able to give him the benefit of the doubt more often than not.
Sterling does not harbor any ill will against Roland, and whenever Sterling wrongs Roland, Sterling views it as something he has to do to appease his sponsor, not as a personal attack against Roland.
Roland does not trust Sterling to begin with, but once Sterling steals his ID, he greatly dislikes him.
If thrown into a room together, they would both probably be okay as they are both good at keeping a calm composure, but neither would enjoy the experience.
Sterling thinks that Felix is 'a wet little sop' and that 'infuriates' him, though he also acknowledges that Felix is a good friend to Sadie and that he cares about her. Sterling does not really dislike Felix too much, he just thinks he is a wimp.
Felix dislikes Sterling and does not trust him. This is due to all of the clearly shady things Sterling does. But most importantly, it is due to his jealousy toward how much Sadie enjoys Sterling's company as Felix likes Sadie regardless of route.
Sterling and Nigel are like brothers. Nigel babysat Sterling, and they were close enough in age that they treated each other as brothers. Their current prank war is a testament to their friendship.
Nigel acknowledges that not all that Sterling does is right, but since he knows a lot about Sterling's home life, he is a lot more understanding than everyone else.
Sterling has nothing against Anna and even mentions how much Anna cares about Sadie.
Anna is in the middle about Sterling. She knows he really is a good guy, but she can't overlook all of the shady things he does and how many times he has hurt Sadie.
Sterling likes Evelyn and thinks that she is really spry for someone her age (though she isn't that old).
Evelyn has heard of Sterling before their first meeting and though she seems a bit apprehensive, she is quick to accept him, but she still clearly has some doubts.
Ruddy is Sterling's 'sponsor', meaning that Ruddy is paying Sterling large sums of money in order to do his bidding. Most of what he orders Sterling to do is illegal, or at the very least morally questionable. Ruddy views Sterling as little more than a skilled pawn, and is more than willing to threaten to kill him. Sterling despises Ruddy for making him hurt Sadie.
As soon as Lorelei and Sterling meet, sparks fly regardless of if Sadie and Sterling are dating. They frequently flirt with each other, and in Sterling's friendship endings, since Sterling is left pinning after a 'blonde, blue-eye archaeologist', he gets with Lorelei, much to Sadie's dismay.