Nagisa's character profile lists Monaca as one of the things he likes the most. In his memoir, Nagisa mentions Monaca is much more important than his own life. They're also holding hands on one of Jataro's art works, meaning he ships the two.
Nagisa was impressed by how Monaca didn't give in to despair after Junko Enoshima's death. He developed a crush on her. This was shown when he was embarrassed to answer Monaca's question whether he likes her, when Kotoko asked him if he's jealous of her friendship with Monaca, and when he told Komaru Naegi and Toko Fukawa that he will not let them hurt "their princess" Monaca.
Monaca eventually reveals her true nature to Nagisa in Chapter 4. She reveals she was never interested in a paradise for children and had no expectations for Nagisa. She then steals Nagisa's first kiss after he slaps her when she was getting close to him. Later, she kisses him once more and tells him she loves how worthless he is. Nagisa is seen broken afterwards, even during his boss battle with Komaru.
In Future Arc, Nagisa tells Komaru not to hurt Monaca because she still used to be their friend.
- “And... I suppose I am a babysitter of sorts for the Warriors of Hope... Monaca excluded.”
- — Nagisa introducing himself as a sort of babysitter for the Warriors of Hope excluding Monaca.
- Nagisa – “Now's not the time for banter. We've got a lot of work to do here!”
- Kotoko – “Oh my oh my, are you perhaps...jealous?”
- Nagisa – “W-What're you talking about? What do I have to be jealous of?”
- — Kotoko teases Nagisa for liking Monaca in Chapter 3.
- Toko – “Basically, Monaca's the mastermind, and to save Master, I gotta deal with her.”
- Nagisa – “Monaca is our princess! I will never allow that!”
- — Nagisa about not wanting Monaca to get hurt.
- Monaca – “You love Monaca, right? You can't live without Monaca, riiight? You love me sooo much, sometimes you even have perverted thoughts about me, riiight?”
- Nagisa – “S-Stop...Please stop!”
- Monaca – “You're glad you got to kiss Monaca, right? If you work hard, I can do other things too, y'know...So you'll listen to Monaca's little request, right?”
- Nagisa – “D-Don't come near me! Get away from me!”
- — Monaca begins emotionally traumatizing Nagisa in Chapter 4.
- Monaca – “You have such a hard time, everyone has such high expectations of you because you're so smart. And you even have high expectations for yourself. And you're always suffering from that, right?”
- Nagisa – “I said, don't come near meee!”
- Monaca – “But Monaca won't expect anything of you.”
- Nagisa – “What?”
- — Monaca exploiting Nagisa's desire to meet everyone's high expectations of him.
- “I need to save Monaca. It's her I need to think about. She is much more precious to me than my own life.”
- — Nagisa about Monaca in his memoir.
- “If Monaka is still trying to do something, I want you to stop her. But don't be too hard on her. She was a friend...”
- — Nagisa to Komaru in the subbed version of episode 7 of Future Arc.
The ship started off rather well. With quite a few hints that Nagisa is attracted to Monaca, it gained a lot of shippers. However, with Monaca acting suspicious and coming off as the obvious main antagonist, some fans were wary and worried about the ship's future.
Eventually, once Monaca started abusing Nagisa on-screen in Chapter 4, using his attraction to her to her advantage and to mess emotionally with him, majority of the ship's shippers abandoned it. It is rare to find fans who'd still ship the two after that moment.
In some fan fictions, fans portray the ship and Monaca herself sympathetically, as a sort of "What if?" scenario if Monaca had never emotionally abused Nagisa. Others stay loyal to the canon portrayal and shows the ship as a tragic one-sided attraction.