All of this happens gradually when you go to "Eve's Place" regularly.
1. She's gonna notice that you gained weight very fast, warning you about "dirty bulking". That event gives you the option to snoop around her office. You can find old photos and some sort of calorie log where you learn that she has a love for chocolate. You need 1 chocolate from All Foods that you can place on her table. After a few more visits, your character will notice that Eve is getting thicker.
2. Explore city to find Bonnie in a candy making course. She will give you some sweets she made herself. Next time you visit Eve's, Eve is gonna notice and you can give the sweets to her.
3. Eve is gonna have a depressed phase, during which you won't be able to train with her. You now have the option of researching fat athletes on your computer, there you will learn about sumo. Then you can purchase a book on sumo at the book store in the mall. Give it to Eve and she will regain her will to train (plus gaining weight). When you're training with her, she's gonna be a lot easier on you now, cause she understands now that you're more into gaining weight than losing it.
(You also should have the option of buying homemade chocolate at the Health Bar by now, they are required for Carmen at the maid cafe)
4. Eve is gonna leave for a bit to visit a sumo training camp, so she will be unavailable for a few days. At some point, you're gonna have an e-mail on your computer where she invites you to a sumo tournament.
5. After the event, you can give the perfect sandwich to both Eve and Bonnie, getting them to their final stage.
(During all of this, the Health Bar is gonna update it's contents to reflect the state of Eve, adding chocolate milk, sweets, and adding weight gain supplemenmts, while also removing the health shots cause nobody uses them)