Nina is the nineteen-year-old neighbor of Sorin. In Act 1, she and Konstantin are in love and she's starring in his play. She's so happy: "My heart's overflowing with you," she tells him (1.42). But even before the play, there's trouble in paradise.
Nina is anxious about the attendance of the famous writer Trigorin at their play. She loves his stories, and as soon as she gets a chance—after the collapse of the play—she tells him so in gushy, gushy terms. First she flatters the man's ability to write:
"I'd think once you've known the thrill of creation, all other pleasures must pale in comparison." (1.139)
It clearly becomes evident, however, that a great deal of Nina's attraction to Trigorin stems from his success in the world. "I'd love to be in your shoes," she tells him, "to see what it feels like to be a famous writer. What does fame feel like? How do you know when you're finally famous?" (2.93). After Trigorin explains to Nina how tiresome writing and fame can be, she still doesn't buy it. Imagining herself as famous makes her giddy.
Somewhere along the way, however, Nina's Trigorin thing transforms from idealized admiration to self-destructive obsession. When they encounter the seagull Konstantin killed, Trigorin pretty directly states his intentions:
"A girl like you […] [is] happy and free as a seagull. Then a man comes along, sees her, and ruins her life because he has nothing better to do." (2.117)
How does she respond to this creepy, creepy comment? When Trigorin and Arkadina are leaving, she presents him with a medallion referencing this quote: "If you ever need my life, come take it" (3.88). Oof. No, Nina. The correct response to an old dude saying something like this to you is "Uh, I have to go. Now." But she offers herself to him, for whatever use he sees fit. This self-sacrifice manages to get Trigorin's attention and they arrange to meet in Moscow.
In Act 4, Nina returns to the house. It's been two years, during which time she had a baby with Trigorin, lost it, and lost him. Her nerves are fraught; she keeps digressing, quoting Turgenev and calling herself the seagull. Life hasn't been kind to her but she's a different woman than she was when they first confessed their dreams of the creative life:
And now I know, Kostya, I understand, finally, that in our business—acting, writing, it makes no difference—the main thing isn't being famous, it's not the sound of applause, it's not what I dreamed it was. All it is is the strength to keep going, no matter what happens. (4.176)
These are the true facts, folks. But luckily for everyone out there (especially the people who think that Johnny Cash's "Ring of Fire" is a song about the joys of being in love painfully), you can read The Seagull and sober up from your love drunkitude... instead of following in Nina's footsteps.