You saw it coming ya'll. Now let's get started.

For starters we have the Squid Sisters! Callie and Marie!

Marie first met Mario when he accidentally stumbled upon one of the kettles that lead to one of the boss fights that agent 4 was in the middle of. Since Mario and Luigi were exploring the sewers to prove to their parents that their dream of being plumbers wasn't stupid, they went down.
But they somehow gotten split up, as Luigi got taken hostage by the Octarions, and Mario ended up where Agent 4 was. After he helped her fight off the boss, he met Marie and soon saved Callie.

Marie fell for him due to his happy-go-lucky personality as well as his need to help others. As he instantly agreed to help her and agent 4 find Callie as he also wanted to save his brother as well. And Callie falls for him after he saves her.

Secondly, we have off the Hook!

Mario met them when he first entered inkopolis square. He helped Marina get back home safely after seeing her being cornered by pervs who wanted to get down and dirty with her. After escorting her home, he met Pearl after saving Marina from her fans. Of course, Pearl didn't really trust him until he proved himself to her when he defended her from fans that simply hated on her. It was then that Pearl realized why Marina spoken so greatly about Mario.

Now we have Agent 4(Kelsey)

Agent 4 met Mario when he saved her life from one of the bosses in the kettles. The two instantly hit off their relationship to a great start and together they both save Callie from Octavio. Though he did save her life more than once and because of that, she fell for him.

Now for Agent3(Emilia)

Agent 3 met Mario during the Octo Expansion incident. She of course distrusted him and even treated him rudely when he met her again after everything calmed down. It wasn't until she saw how mad he gotten after agent 8 got hurt that Mario wasn't a bad person, and she realizes that she has caught feelings for him after their first hangout. She also finds it weird how he is able to swim but has trouble going into his squid form.

(Can inklings swim? I know they dissolve in water, so how the hell do they take baths?)

Agent 8(Veronica)

She is a bean with an innocent mind, just like Agent 3, Agent 8 met Mario during the Octo Expansion incident. She finds him to be pretty friendly and enjoys his company, and she doesn't know what love is, but she does know that whenever she is around him, she gets a happy but weird feeling. After all, she took a bullet for him as well.

Yumiko

She is a member of the Dark Inklings, she meets Mario during the Kyoto incident. Once she realizes that Mario has darkness deep inside of him as he also has the ability to absorb shards of the dark star into him. She takes a mild interest that soon turns into affection for him, as she turns against the others to protect him.

That's all for now. Until then, I'll see you guys later!!!