Relations

From friends to comrades to enemies, Cait Sith and Reeve have all aforementioned in spades—not mentioning those alluded to switching hats at various points throughout the game.

CloudStrife

Cloud's initial impression of Cait Sith was much the same as everyone else's: who's this suspicious goof and what does he really want? When Cait Sith's real intentions are discovered, Cloud looses all and any trust in the robotic cat, and no matter what Cait Sith does to try to do to prove his genuineness, Cloud refuses to "fall for it". The only reason Cloud allows Cait Sith to stay in the party is because he's kidnapped Barret's daughter Marlene. As Reeve's dedication to Shinra wavers, Cloud begins to place a little more trust in him and his robotic counterpart, including counting on Cait Sith for inside information on Shinra whenever possible.

AerithGainsborough

In the beginning, these two appear to get along quite well. After it's revealed Cait Sith is a spy for Shinra, however, Aerith grows a bit colder toward the fortune teller. Later, when Cait Sith No.1 decides to sacrifice himself to get the Black Materia for Cloud and company, Aerith finds it quite admirable and encourages him with a saying usually reserved by Tifa: "Be strong!" This gives Cait Sith the motivation he needs to sacrifice his body:

She told me to 'Be strong'. I feel so happy.

Quoted by Cait Sith

Although a second Cait Sith does come along, any friendship that could have developed between him and Aerith is prevented by Aerith's death. Reeve is the one who informs Elmyra of her daughter's death, a hard task for anyone to do.

TifaLockhart

Tifa, being both optimistic and one prone to giving others the benefit of the doubt, appears to have the most faith in in Cait Sith. Still, Tifa doesn't agree with Cait Sith's methods, and like the rest of the group, has inhibitions with him joining them on their journey. As is her nature, she realises the futility in arguing, avoiding conflict with Cait Sith whenever possible. She doesn't fully trust him, even after Cait Sith helps save her life at Junon. Despite this, Cait Sith appears almost to care for Tifa. Or at least thinks she's hot, as he sometimes says things like this:

If Tifa were my nurse, I'd switch places with Cloud any day!

Quoted by Cait Sith

More seriously (and to the point), when Cait Sith predicts Cloud and Aerith to be a perfect match, he feels poorly about it, knowing Tifa's in love with Cloud. He sympathises with her position, and attempts to help her out whenever he can. After Cloud disappears into the Lifestream and Tifa becomes extremely depressed, Cait Sith promises to provide her with any inside information he can.

BarretWallace

These two never got along! Barret always felt something was fishy with "that no-good cat!" He's also the first to suggest getting rid of Cait Sith for good when they find out he's actually a Shinra spy, and is rightly furious when Reeve uses his daughter Marlene as a trap to keep Cait Sith in the group. On the same token, when Cait Sith first encounters Barret, he's shocked that such a dangerous-looking, intimidating guy could be part of Cloud's party. After much uneasiness and bickering, the two are eventually forced to agree to a truce in order to rescue Tifa at Junon. Afterward, Barret continues to bug Cait Sith about his being a spy. Things come to a head between the two when Reeve, through Cait Sith, attacks what AVALANCHE claims is their right to destroy the reactors, even at the cost of people's lives:

Cait Sith: "I been itchin' to say this to ya fer a while now!
    When ya blew the Midgar No. 1 up, how many folks d'ya think died?"
Barret: "...That was for the life of the planet. Ya gotta expect a few casualties."
Cait Sith: "A few? Whaddya mean 'a few'? What may be a few to y'all is everythin' to them who died......
    Protect the planet. Hah! Y'all sure sound good! Ain't no one that'd go against ya.
    So ya think ya can do whatever y'all want?"
Barret: "I don't want hear from no one in Shinra..."
Cait Sith: "......nuthin' I can do 'bout that...

Barret and the others are surprised to hear this from Reeve, a Shinra employee. They probably believed a side like this didn't exist; after all, care of and concern for the lives of others was exactly opposite of what everyone in Shinra had proved themselves to cherish. Some trust builds after Reeve's revelation, and this small trust becomes essential when the planet is seriously threatened and everyone—Barret and Cait Sith especially—have to work together to prevent its destruction.

PresidentShinra

The first President of Shinra seems to have some semblance of respect for Reeve. When Reeve is stressed due to the reactor bombings and general uncooperativeness from his coworkers, the President allows him to take a vacation (something a good boss would do, no?). He also divides the 15% Mako increase between Reeve and Scarlet, despite Palmer desperately needing the extra funds, ultimately aiding in Reeve's efforts as head of Urban Development and thus aiding the people. However, the President rarely takes into consideration anything that Reeve says. Reeve is quite obviously has misgivings about the President's choices, and likely the only reason he'd remained a member of the main committee was because of his position as head of the Urban Development Department—the President had recognised his value in such a position and hadn't yet been so corrupted by power as to ignore make the choice of removing him from that position.

Heidegger

Heidegger is the type of person who commands agreement from everyone in every way. Reeve and he often clash because of their very different ideals, as well as their mutual inability to back down. Heidegger usually shoots Reeve's concerns down before anyone else can give input, much to Reeve's personal chagrin. When both Presidents are killed and Heidegger and Scarlet take over Shinra, they arrest and exile Reeve without a second thought. (Reeve, for his part, isn't exactly surprised by this.)

Scarlet

Although not as harshly at odds with Reeve as is Heidegger, Scarlet doesn't care much for her coworker. Working with Heidegger, she later has Reeve expelled from Shinra. Of her, Reeve has this to say:

... I hate that broad

Quoted by Reeve

And that pretty much sums up Reeve's feelings about her.