The Flash and (Green) Arrow. In the comics, Barry and Oliver didn't even know each other until they were both established heroes and Justice League members. Due to their very different backgrounds, they were never that close with their friendship with Hal Jordan/Green Lantern serving as their main point of contact; in fact, Depending on the Writer the two actually had quite an antagonistic relationship born out of petty jealousy and contrasting political ideologies (Barry, being a cop, was fairly conservative in his views). Whereas in this continuity, they had a pre-existing relationship based on a past meeting prior to Barry gaining his powers. It is Oliver who most directly inspires Barry to become a masked superhero, and he even provides the inspiration for the name 'Flash'.
Kazumi Adachi is Emiko Adachi mother here, but in the comics she's her maternal grandmother.
Barry and Wally sometimes refer to one-another as brothers, owing to the fact Barry is both his brother-in-law and his foster brother. In the comics, Barry was explicitly Wally's uncle, and the two viewed one-another as more akin to father-and-son. Even Wallace West -the Decomposite Character introduced in the wake of the show to adapt its version of Wally- is explicitly a surrogate-son to Barry.
Barry Allen has virtually no relationship with Vibe and Caitlin Snow in the comics (comics Caitlin Snow in particular wasn’t even introduced until shortly before the show aired),:while in the show they were made his close allies from day one. Caitlin Snow remains his ally as of Season Eight, while Vibe departed on good terms.
Thanks to time travel, Eobard Thawne becomes a mentor figure to the Flash, Vibe, Killer Frost, and XS.
Comics Thawne had little interaction with his ancestors. This Thawne openly despises Eddie and singles him out as the "loser" of the family.
The comics Thawne was a Stalker with a Crush for Iris, trying to force her into marriage and killing her when she refused. The Arrowverse Thawne has virtually no interaction with Iris, at least until Season 8 where he takes over Barry’s life and tries to marry her.
Jesse Wells
About the only canon relationship that stayed more-or-less the same was Jesse Wells's relationship with Jay Garrick, and even that isn't quite as close. Specifics:
Her father; Johnny Chambers was a loving father to Jesse and she was an utter Daddy's Girl, though his sport coach-like parenting left her with some issues with earning approval. Here, she's repeatedly Calling the Old Man Out over his dysfunctional personality and terrible people skills, and while he loves her, he struggles to show it. Also, her mom is a case of Deceased Parents Are the Best when in the comics, Libby Lawrence was a smothering, somewhat overbearing mother who survived her ex-husband by many years, and Jesse had a troubled relationship with her. In many ways, the dynamic between them switched.
Wally West; in the comics, Jesse was one of Wally's partners in crime-fighting but outside of All Love Is Unrequited on her side, they were Just Friends, with Wally seemingly seeing them as Like Brother and Sister. Wally was also much more powerful and far more experienced than Jesse, causing her to often feel like she Can't Catch Up, while Wally had a Big Brother Instinct and constantly went out of his way to help the otherwise friendless and constantly overworked Jesse socialise more. The show, obviously, made them a Beta Couple and had Jesse be the one who got powers first, helped train Wally, and caused him to feel insecure about his abilities next to hers, especially as she was a Teen Genius from a rich family.
Barry Allen; Jesse in the comics had almost no relationship with Barry Allen, introduced while he was dead and being Demoted to Extra alongside most of the Flash Family shortly after he came Back from the Dead, and in general she was more a friend-of-a-friend. While they're still not particularly close in the show, Barry helped train her and seemed to take over Wally's role of being Jesse's Big Brother Mentor, and Jesse is now in-universe Barry's counterpart as the Flash of Earth 2.
Wally West/Kid Flash
Linda Park, Hartley Rathaway, and Roy Harper are among Wally's closest associates in the comics- Linda being his wife and Hartley and Roy being two of his best friends. In the Arrowverse, Wally has yet to interact with any of them.
In the comics, Magenta/Frankie Kane is Wally's Psycho Ex-Girlfriend. Here, that's not the case, likely due to her Age Lift.
Comics Zoom is Wally’s Evil Former Friend, with Wally seeing his fall from grace as his greatest failure. Here, Wally and Zoom never were friends to begin with.
Fred Chyre in the comics was a major part of Wally's supporting cast. Here, Chyre dies a full season before Wally is introduced and they never meet as a result.
Hartley Rathaway comics counterpart is one of Wally West's best friends. They haven't even met yet in the Arrowverse.
Wally West is Iris's younger brother instead of her nephew due to him being a Composite Character of comic Wally and Daniel West, Iris's brother from the New 52 continuity. In addition, Wally's relationship with Linda Park was removed in favor of her being a minor love interest for Barry.
Sue Dearbon
A rather unusual and unplanned example that happened because of Real Life Writes the Plot. In the comics, Sue Dearbon became Sue Dibny pretty much right away, as she married Ralph Dibny/Elongated Man at the beginning and they remained Happily Married until Sue’s death in Identity Crisis (2004). In the show, Sue Dearbon was introduced in a storyline involving the already-introduced Ralph Dibny, and while their relationship had more snark in it (owing to Sue becoming a spy-style character and Ralph becoming a Composite Character with Plastic Man), it did seem that the show eventually planned to bring them together. But then Ralph’s actor, Hartley Sawyer, got fired over controversial tweets. Ralph was subsequently dropped from the show, while Sue makes sporadic appearances season-to-season, having become her own character who is unlikely to take the surname ‘Dibny’.
Overlaps with Adaptation Name Change. In the comics, Bette Kane is a cousin of Kate's. On the show, however, Mary Hamilton, a character loosely based on Bette, is Kate's step-sister.
Poison Ivy (Pamela Isley) has a much better relationship with her family than her comics iteration, as their sufferings provide the backbone of her Freudian Excuse.
