Luigi

Luigi is Mario's younger, taller, and slightly thinner twin brother. Luigi has assisted and fought alongside his brother on many occasions. He makes is Super Mario Bros. X debute in Super Mario Bros. X: Alternate Kingdom, where he left the Mushroom Kingdom to the Eleanor Kingdom with Mario. Having heard the kingdom was in danger, the Mario Bros. decided to save it. In Super Mario Bros. X: Sea Side Adventure, Luigi left the Mushroom Kingdom with his brother Mario to Sea Side to search for their missing friends (Yoshi, Toad and Princess Peach), save the kingdom and its two monarchs and defeat both the Koopa Troop and the 8-Bits.

Invasion of Sea Side
It was implied that Luigi, along with Mario, Yoshi and Princess Peach came back to Sea Side to visit King Varun and retrieve their missing companion Toad at the ending of Invasion of Sea Side.

Alternate Kingdom
Several years had passed after the ending of Invasion of Sea Side. During the beginning of Alternate Kingdom, Mario and Luigi were all sailing on a boat. During their adventure on a boat, there was a huge storm and wiped them out. The Mario Bros. was found unconcious on an unknown world. After waking up, they met a mysterious man named Marvin, who was really worried about something. He requested the brothers to help him save the woman he had fell in love with and save the world from a peril. The brothers accept and begin their new journey on an unknown world.

Sea Side Adventure
Several months later after the events of Alternate Kingdom, Luigi went on a new adventure on Sea Side with Mario.

Mario
Mario is Luigi's older twin brother. They ventured together for several years, even in the Eleanor Kingdom. After learning that one his friends have gone missing in Sea Side to meet the king, Luigi and his brother have departed the Mushroom Kingdom to Sea Side to rescue them. The two of them have ventured through many landscapes of the Sea Side Kingdom to find Toad, Princess Peach and Yoshi. The Mario Bros. care deeply about each other and always team up together, while the second one Mario and/or Luigi often team up with being Yoshi.

Princess Marina
Princess Marina also have admired Luigi for his bravery to save the kingdom of Sea Side with Mario. Since she had scared him, she believed that she was wrong about him, losing her respect for the hero. Despite that, Marina believe that Luigi would be strong and cool like Mario, though she hasn't shown that to him. Luigi seems to have a crush on Princess Marina, though she is oblivous and has feelings for Mario instead.

Yoshi
Luigi is Yoshi's second best friend, when the first is Mario. Luigi often teams up with Yoshi and fight together as a great team. The two of them always assist Mario as well. They are often known as the "Green Team" due to their colors.

Marvin
Marvin is Luigi's counterpart. These two share a lot in common, except that Marvin was more of a coward than Luigi several years ago. Since Marvin had became friends with Varun and Doshi again, he became more confident and courageous. Marvin also taught Luigi to stay strong, confident, courageous and watch over his older brother no matter what the odds are. Luigi also supported Marvin to find Princess Apple with him.

King Varun
King Varun views Luigi as a hero and admires him. He compare him to his older brother Mario because of their bravery and they both defeated Wendy O. Koopa, Boom Boom and the Koopa Troop soldiers who were with them and saved the Sea Side Kingdom. Sometimes, Luigi mistakes Varun to be his older brother or Mario, yet he still questions himself if he's somehow related or not because of his main resemblance to Mario. Despite that, Luigi respects him and treats him like a brother.

Goldian Duck
Luigi was one of Goldian Duck's nemesis, when both Mario, Marina and Varun were his real enemies. However, Goldian doesn't hold any grudge against Luigi, but Luigi does because he often fights with his older brother and his companion Toad. Despite that, those guys get along very well and he is being nice to Luigi, just as he's being kind to Goldian Duck. He is also very protective for Luigi. Luigi is the second person who is also able to calm Goldian down, while Princess Peach is the first.