I agree. It was pretty obvious that Saix was irked over Axel's friendship with Roxas and Xion, ESPECIALLY Xion. You could tell he found it utterly ridiculous that Axel would befriend someone who wasn't even real in a sense.
For someone who apparently cannot feel (as he convinced himself always again) that is either pretty ironic or simply idiotic.
That's also something that lies solely in the viewpoint of the characters, as she only wasn't real in
Saix' opinion/perspective, and I do not recall him having the monopoly on deciding who is real and who not. To both Axel and Roxas Xion
is real and that's also why they see her actually as a person.
The Riku Replica is practically the same issue, as Larxene only saw him as "a toy" and Riku as a "fake" while Sora saw him as his own, real individual.
All in all, Saix is pretty ignorant and selfish, and as Spockanort pointed out, reminding Axel of their "friendship" and calling him Lea only served to bind Axel to him and the plan, not because Saix truly cared about Axel
or their friendship. One of the main reasons Axel eventually turned to Roxas (and later on Xion) was because when with them he actually felt like he had a friendship, while with Saix it was only some superficially surface relationship that was based on old memories, but no more actual substance.
A friendship needs to be nurtured and maintained from both sides, not just one.
The fact that Axel feels no more need to confide things with Saix, yes, even the opposite as in that he thinks it's best to hide several things from him, speaks volumes on how their relationship had really detoriated.
From what we see throughout the entirety of Days, even before Xion and Roxas truly bond with Axel, is that Saix treats Axel more as his servant rather than a friend.
It is interesting to observe all this and then realizing how far Saix/Isa really fell into the web of lies that Xemnas and Xigbar had weaved as part of Xehanort's plan and how ignorant he became of things.
In a meeting between just Xemnas, Saix and him, Xigbar even
mocks Saix' lack of clarity
and his failure to see that Xion developed an independent sense of self (and thus a heart, but that is kept silent cause it's part of the plan):
Days said:
Xemnas: What else?
Saïx: Somebody accessed our main computer without authorization.
Xigbar: "Somebody"? Are we all gonna pretend we don't know who? Little Poppet
is turning into a problem--and I think you catch my meaning.
Saïx: Nonsense. I see no problem whatsoever.
Xigbar: Ha ha! Well no, apparently you don't!
Saïx: Something you find amusing?
Xigbar: If people see with their hearts, Saïx, then you're even blinder than
the rest of us.
Xemnas: Our plans remain unchanged. Axel, Roxas, and Xion will play the roles
Kingdom Hearts has chosen for them.
Saïx: But sir, Xion... If we don't take steps--
Xemnas: Take steps? How can you not see how perfect this is? Xion is marching
right into the arms of destiny. The only steps we need take are two: watch...
and wait.
Xigbar being the troll he is, gives actually a blatant hint there without the danger of anyone catching on.
Xemnas' statements can come over as complete nonsense, or also as hints towards what's really going on.
Xehanort seeks to become one with Kingdom Hearts and thus become a god, so Xemnas saying that " Axel, Roxas, and Xion will play the roles
Kingdom Hearts has chosen for them." can actually also be read as " Axel, Roxas, and Xion will play the roles
Xehanort has chosen for them."
Sure, Xehanort doesn't have the power yet, but he
does play around with people's destinies and designates roles for them in his web of plans that will eventually lead to his goal, the X-blade and after that KH itself.
His second mumbling about "marching into the arms of destiny" can also have a second reading in that it is Xehanort's way of saying "I love it when a plan comes together" (Blatant A-Team reference, lol) or, like Emperor Palpatine: "Everything is proceeding as I have foreseen."