‘Acting as Bridge’ to Lovers

A main lead acts as a “bridge” (help two people get together) to the other main lead and another character but ends up being the lover instead.

FL "helps":

• The Romance of Tiger and Rose - The FL is a script writer for a drama who ends up in the world of a script she is writing, in the role of a secondary female character / the sister of the female character who was originally the lead in her script (I'll refer to her as the SFL). The actual FL tries to get the ML together with the SFL like they were supposed to be in her script, despite the fact that she (the FL) is married to the ML, and the fact that the ML has fallen for her. Eventually the FL falls for the ML before returning to the real world, where the actor who plays the ML wakes up in the hospital remembering everything that just happened as a dream (i.e., the ML and FL both remember falling for each other in the script/dream world and are together in the end).
• She Was Pretty - According to the narrative, the FL was attractive as a young girl and became unattractive as an adult, while the opposite was true for the ML (unattractive --> attractive). Embarrassed to have the ML meet her again with her changed looks, the FL has the SFL take her place. The SFL and ML end up dating (without the FL knowing) with the ML thinking the SFL is the FL he knew as a kid. Eventually the ML and FL end up together.
• Love Between Fairy and Devil - The ML, FL, SML and SFL are immortals. The ML and FL enter the mortal realm together and retain their memories. There, they encounter the SML and SFL, who are undergoing a trial and do not have their memories. Thanks to the FL's actions, the ML ends up engaged to the SFL and (despite the ML's best efforts) the FL ends up engaged to the SML. Joint weddings are held, but the wedding rituals are not completed and of course the ML and FL later end up together.
• Mr. Queen - This one is super complicated to explain but I'll give it a go. KSY is a noble woman in Joseon engaged to the King. Before her wedding, she nearly drowns and ends up in a coma. When "she" wakes up, her body has basically been possessed by the soul of JBH, a male chef from modern-day Korea whose soul travels back in time after JBH has an accident that puts him in a coma. After some time passes, JBH's inner voice turns into KSY's inner voice and he can remember KSY's memories. He also seems to be affected by KSY's feelings for the King. Despite his marriage to KSY, the King is in a relationship with JHJ at the beginning of the drama. It turns out that KSY saved the King's life when they were children, and JHJ took the credit for it, so KSY was sort of a bridge for JHJ and the King getting together. Then for a second more complicated bridge analogy, the King falls for JBH/KSY which is only possible because JBH's soul is in KSY's body, and KSY's feelings also play a role in the King and JBH/KSY getting together with mutual romantic feelings. JBH/KSY later takes a bullet for the King and this results in JBH's soul returning to his body in modern times, leaving only KSY's re-awakened mind / soul in her body in Joseon. So the King and KSY end up together.

ML "helps":

• Boy for Rent - The FL has a crush on a guy who rejects her due to her lack of maturity / experience. The FL hires the ML to teach her / help her gain relationship experience. I don't actually remember exactly how it ends, but I'm pretty sure the ML and FL end up happily together.
• I Need Romance Season 3 - He's not super happy about it, but the ML helps the FL realize her feelings for the SML. She later dates the SML for a bit while stringing the ML along. The ML eventually gives her an ultimatum (a rare case where it's totally justified) and in the end the FL chooses the ML.
• Wednesday 3:30 PM - Not quite what you described (the ML isn't really acting as a bridge) but it's somewhat similar. The FL's boyfriend dumps her and starts going out with their mutual colleague. The ML helps the FL make her ex jealous / regret dumping her, and of course the ML and FL end up together.

HIStory 4: Close To You

Enchante

The 2nd couple in "TharnType 2" becomes leads in "Don't Say No" and fit the prompt.

I am realizing that most of the recs I know are either outside of dramaland or with a secondary couple...