Beep-0 (pronounced "beep-oh") is an ally character introduced in Mario + Rabbids Kingdom Battle. He is a small robot assistant with blue "eyes" and rabbit ears, and serves to guide the playable characters throughout the levels. Beep-0 provides commentary throughout the game while helping the team search for the Rabbids fused by the SupaMerge (with his color changing depending on his emotions), and is absent during battle sections. By default, Beep-0 does not have rabbit ears or the rivet on him that resembles a Rabbid's teeth, only gaining them after entering the Mushroom Kingdom.
Beep-0 is first seen in the introductory cutscene of Mario + Rabbids Kingdom Battle, inside the Genius Girl's workshop as her assistant. The Genius Girl is working on the SupaMerge, a headset with the function of fusing two things together, which she hopes will solve an energy crisis. However, after experiencing an overheating problem with the headset, she leaves to take a break. Soon after, the Rabbids appear in the workshop, having been transported there by the Time Washing Machine. As the Rabbids create mischief, one of them finds the SupaMerge and puts it on. Unable to control the headset's powers, this Rabbid fuses other Rabbids with several objects in the room, creating Rabbid Peach and Rabbid Mario in the process. The Time Washing Machine is then hit by a beam from the headset, causing Beep-0 and the Rabbids to be transported to the Mushroom Kingdom.
In the sky above the Ancient Gardens, Beep-0 gains rabbit ears, while the Rabbid with the SupaMerge headset's appearance changes as well. Beep-0 is almost hit by a falling Time Washing Machine, but is saved by Mario, who takes him to safety. Upon landing, they soon encounter Rabbid Peach and Rabbid Luigi, who join them and help them fight off a group of villainous Garden Ziggies, which they successfully do due to weapons being sent to Beep-0 by a sender called "F.B.". Soon after, they look upon a vortex called the Megabug, which Beep-0 states is what brought him and the Rabbids to the Mushroom Kingdom, commenting that it has grown in size. They later come across the Rabbid with the SupaMerge headset and Luigi. Both find a Rabbid sleeping on a Piranha Plant, and are scared by it. The SupaMerge Rabbid, as a result of the fear, unintentionally fuses the Rabbid with the Piranha Plant, creating the Pirabbid Plant. After the plant is defeated by Mario's group, Luigi joins them. They later scale a large tower of blocks and find the SupaMerge Rabbid again, scaring him. Taking advantage of the situation, Bowser Jr., who observed what happened to the Pirabbid Plant, flies in on his Junior Clown Car and befriends the Rabbid, naming him Spawny. Intending to use Spawny's powers to take over the Mushroom Kingdom while his father is on vacation, Bowser Jr. flies off, leaving Mario's group to fight Rabbid Kong. Mario's group defeat Rabbid Kong, and return to Peach's Castle to report their progress to Princess Peach.
Beep-0 then joins the group as they journey to Sherbet Desert. About midway through the desert, they defeat Blizzy and Sandy, after which a recently freed Rabbid Mario joins them. They later come across the Icicle Golem and prepare to attack it, but are frozen. Just then, Peach flies in on her parasol and frees them, joining their active team as they defeat the monster.
They then travel to Spooky Trails, where they are directed by a mysterious third party to find the three "Relics of Goodness", which they claim will awaken "Tom Phan"; a being that has the power to defeat Bowser Jr.. After finding the three relics, they are told that a full moon is required to awaken the relics' power. While finding a way to bring back the full moon, they encounter Calavera, holding Rabbid Yoshi hostage. After Calavera's defeat and Rabbid Yoshi's joining of the team, they venture further and encounter fortune-teller Madame Bwahstrella, who says that they are nearing their "doom". Beep-0 refuses to believe her however, not believing in superstitions. After reverting the full moon, Bowser Jr. appears with Spawny, revealing that he was sending the messages relating to "Tom Phan". He then scares Spawny, causing the latter to fuse a Rabbid with the three "Relics of Goodness", creating the Phantom. After the Phantom sings a song, he is defeated.
The group then travel to the Lava Pits. Here, they face off against more enemies before encountering Bowser Jr. and Spawny in the former's Mecha Jr.. Bowser Jr. is defeated and flees as Mario's group is joined by Yoshi, who has rescued Spawny. They attempt to leave the pit with Spawny. However, after getting Spawny past some enemies, he becomes attracted to the Megabug, which has grown large enough to obtain a physical, phoenix-like form. The Megabug consumes Spawny and flies off to Bowser's Castle, creating more Rabbid-fused enemies to halt the heroes' progress. Bowser Jr. returns to tell off Mario's team for allowing Spawny to get kidnapped, noting that at least his father is still on vacation. However, Bowser Jr. gets a text message from his father, saying that the latter has decided to come home early. Realizing his father is in danger, Bowser Jr. flies off to the castle, and Mario's group go there as well. Upon reaching the castle, Bowser Jr. requests the heroes to help his father, as he is barely conscious and near the Megabug. Although initially refusing to help Junior, Beep-0 soon agrees as the Megabug fuses with Bowser, creating MegaDragonBowser. After MegaDragonBowser is defeated, the Megabug separates from the Koopa King, and is attacked by Mario's group, causing it to be destroyed and seemingly closing the rift between the Mario and Rabbids worlds. Bowser Jr. attempts to convince Mario's group to be chastised in his place after Bowser regains consciousness, but Beep-0 returns with the others to Peach's Castle to notify the civilians of the good news.
At Peach's Castle, a new statue of Rabbid Peach is inaugurated, and the overjoyed Rabbid takes a selfie with the group. Meanwhile, Beep-0 sends a letter with weapons to his past self through the Time Washing Machine, revealing that "F.B." is actually "Future Beep-0".
Donkey Kong Adventure
Beep-O reprises his role in the Donkey Kong Adventure DLC. In the DLC's story, he had returned to Peach's Castle with Mario, Luigi, Rabbid Luigi, and Rabbid Peach just after Rabbid Kong and the others had been sucked into the Time Washing Machine. He is the one to get to Rabbid Peach before the machine warps them to Banana Lagoon.
After falling away with only a part of the Washing Machine, they awaken to find themselves under attack by Tropical Ziggies. Having left her weapons back in the Mushroom Kingdom, Rabbid Peach and Beep-O are almost done for, but at that moment, Donkey Kong appears and defeats them, with Rabbid Cranky not far behind. Beep-O then learns from Rabbid Cranky that the Time Washing Machine parts had been scattered all across the island, and that they must get them back to return to the Mushroom Kingdom.