Yoshi

Yoshi (sometimes specified as Green Yoshi) is one of the heroes of Mushroom World and the best friend of Mario and Luigi. He is a member of the Yoshi species and has aided his brethren in saving their homeland on multiple occasions. Yoshi has also saved other kingdoms from the villain Bowser as well. In Super Mario Bros. X: Sea Side Adventure, Yoshi left the Mushroom Kingdom with Princess Peach and Toad to go to Sea Side to meet the king and the princess. However, they had been missing and got caught by the 8-Bits and the Koopa Troop, so it's up to Mario and Luigi to save them.

Invasion of Sea Side
It was implied that Yoshi came to Sea Side for the first time and meet King Varun. He insisted to join Mario, Luigi and Princess Peach to go retrieve their missing companion Toad at the ending of Super Mario Bros. X-2: Invasion of Sea Side.

Alternate Kingdom
Yoshi is set to appear in Alternate Kingdom. He was found in the Eleanor Kingdom's water-themed region for unknown reasons prior the events of Alternate Kingdom. He officially befriended Doshi, his counterpart. When suddenly, the region was taken over by the Blaize Army. Doshi told Yoshi to evacuate then ran away from trouble. Yoshi was then captured by Blaize and got sealed within a ? Block (in a similar fashion to Super Mario World). After the invasion, the watery land got cursed.

Mario, Luigi, Toad and Marvin managed to rescue Yoshi from that ? Block and told everyone what happened. Marvin, being concerned a lot for Doshi, insist on searching for him there.

Sea Side Adventure
Several months have passed after the events of Alternate Kingdom. Yoshi is one of the main protagonists of Sea Side Adventure. At the Mushroom Kingdom, Yoshi and Toad decided to join Princess Peach to go to Sea Side after recieving the message from King Varun that tells the kingdom is in chaos. Just as Mario and Luigi went to the castle, the three were already gone.

Before Yoshi was found by Mario and his friends, he was found on Yoshi Land. He managed to wield a legendary blade his ancestors weren't able to master, the Egg-Blade. He was considered as the leader of Sea Side's Yoshi race. When suddenly, Wart, the 8-Bit Club and the Koopa Troop came to that region and invade it. Yoshi was still no match for Wart and got sealed into a ? Block again, just like in Super Mario World, New Super Mario Bros. Wii and Alternate Kingdom.

Mario
Yoshi is Mario's best friend. Those two often travels together to save Princess Peach and defeat Bowser, along with Luigi sometimes. Yoshi has always looked up to Mario like a hero and a close friend ever since he met him, and Mario looks up to Yoshi in the same manner. Yoshi occasionally joins Mario on his adventures, serving as his loyal steed who carries him across certain obstacles that Mario himself would not be able to get pass. He will always give Mario a ride no matter what. Throughout the years, Mario has gained a strong trust over Yoshi because of Yoshi's loyal and friendly personality. When Yoshi was missing in the Eleanor Kingdom and in the Sea Side Kingdom, Mario came to rescue him and Yoshi joins him. They often team up together and have an amazing teamwork.

Birdo
Yoshi and Birdo were great friends and were often together throughtout the years. Even though their relationship has been quite unclear, Birdo shown that she really liked him in Sea Side Adventure. Yoshi was really shocked to know that she returned back with King Wart since he used to be an old enemy of the Yoshis. Birdo still worries about him, despite that her real target was actually Yoshi so he can help her assist Wart and forgive him for what the frog king had done. Those two often fight against each other throughout the story, but Yoshi usually win.

Luigi
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.

Princess Marina
Yoshi was distant from Princess Marina ever since he helped Mario and others saved King Varun and the castle. She occasionally says that she wants a Yoshi who is just like Mario's Yoshi (despite they are actually friends rather than owner and pet). Unlike how Marina treats Goldian, she treats Yoshi rather like a partner which makes him more confortable being around her. Like Mario, Luigi and Toad, Yoshi worries so much about Marina, just as how Marina worries about him, showing to be more than simple friends.

King Varun
King Varun has a Yoshi partner and claimed that Yoshi reminds him of his partner, despite that Yoshi is nothing like him. Varun recalls to have known Yoshi several years ago since he came with Mario, Luigi and Princess Peach on Sea Side the second time to pick up Toad after the events of Super Mario World. In Sea Side Adventure, Varun and Yoshi have became great friends as the story goes on.

Goldian Duck
At first, Yoshi joined the heroes to fight Goldian Duck, but personally don't hold a grudge against each other. Goldian is the only one who couldn't understand Yoshi well, despite that he has already found out that he can actually talk from Mario and others, which makes him bother Yoshi so much until he says a word. They never team up so often, though both understand each other about what it feels like to be treated like an animal rather than a partner. Just like he did to Mario, Luigi and Varun, Goldian Duck calls Yoshi "old" because of they actually have about the same age.

Trivia

 * The developers of Super Mario Bros. X: Sea Side Adventure expect Yoshi to serve as one of the comedic foil in the series like Luigi (similar to that of Super Mario Bros. Z) and Goldian Duck.
 * Yoshi is one of the fast-type characters.