"Flute" (Big Brother Stitch, Sutitchi Onii-san, スティッチ兄さん) is the twelfth episode of Stitch!-Best Friends Forever. It first aired in Japan on October 19, 2010 and in English on February 16, 2013.
Yuna and Stitch are running across the beach and having fun when they see a box washed up on the beach. Inside they find a furry yellow creature holding a flute. The creature wakes up and says that he can't remember anything except that he was thrown overboard. Yuna and Stitch offer to help him; at first he declines, but he soon grows weak from exhaustion and hunger and they take him back to their house. After the three have dinner, the yellow creature decides to thank them by cleaning the dishes and the sink. Yuna offers for him to stay at their house for a while and he happily accepts.
Later, the three are watching TV when Tigerlily comes home. The yellow guest calls her overwhelmingly beautiful, which of course flatters her and she agrees to let him stay. As the group wholeheartedly laughs, the being sinisterly chuckles to himself.
In his flying base, Hamsterviel brags to Delia that he's sent an extra-ordinary, extra-powerful, extra-transmutant experiment. He explains that the one he sent, number 145, is designed to be an excellent liar and gains power from the attention and adoration he gets, even enough to be as strong as Stitch. Hamsterviel assures Delia that this won't fail as he's cute, convincing, and even plays the flute.
Speaking of flutes, the yellow creature plays his flute to Yuna, Stitch, and Tigerlily exceptionally well, and Yuna decides to name him Flute. Tigerlily criticizes this name, but the being loves it, and happily declares his name to be Flute. He then starts playing again, and Stitch begins stomping his foot to the rhythem. Flute is distracted by this, and seeing it, Yuna and Tigerlily shush him.
The next morining, Flute calls Stitch "big bro", considering him to be family and Stitch agrees to the nickname. Flute offers to drive Yuna's bicycle to school for him. At school, Flute impresses many other students in the school, including Hiroman with an impressive soccer kick, Jessica by laughing so hard at one of her jokes that his jaw unhinges, and Dolores by writing complex math equations.
He continues to dazzle the whole school by playing his flute. Jumba and Pleakley arrive and see him. Jumba slightly recognizes him, but he's not sure from where. As everyone's cheering for him, Flute's arms briefly grow more muscular, but nobody notices.
In the bunker where Gantu and Reuben are staying, Hamsterviel (on the other side of a monitor) points to a picture of Flute and laughs at how even Jumba can't recognize who he really is. He then shows them a picture of a large, ugly green monster slightly resembling Flute and tells them that's what he used to look like before he was transmutated into his currently small and cute form in order to gain more popularity and thus more power. He orders Gantu and Reuben to help Flute become even more popular.
That evening, Yuna, Tigerlily, Stitch, and Flute have dinner and as Stitch is about to eat, Flute snatches Stitch's meat with his long tongue and eats it. Stitch gets angry, but Flute feigns innocence and gives Stitch his meat. Yuna says Stitch could learn a lot from him, as Stitch gets annoyed and Flute gives him an evil glare.
Later in the night, when everyone's asleep, Flute wakes up from his spot on the couch. Stitch hears something and wakes up as well. He goes down to investigate to find Flute greedily eating everything in the refrigerator. Stitch demands to know who he really is and finds out that he's one of his cousins, except evil. Flute smiles and suddenly grasps his arm, falling down and crying. Yuna and Tigerly wake up and come down. Flute says that Stitch hurt him for no reason, much to his shock. Despite Stitch's protests that he's lying, the two don't believe him and make him sleep outside. Yuna lets Flute sleep on her bed (where Stitch usually does) and the evil experiment grins as his arms briefly get stronger once again.
The next day, Gantu and Reuben go to Yuna's school posing as humans and say that they're from a charity organization who plan to sponser free concerts featuring Flute all over Japan. Mr. Matsuda agrees to host a concert. In the school's music room, Flute says that he remembers a little about his past: he was a poor child and had to work in the salt mines for one penny a day. His only possession was a harmonica he found in the gutter. He then proceeds to play said harmonica, and everyone loves it. When he's done, he leaves the room and grows taller and more muscular, having obtained the power he needs. He finds Stitch and easily defeats him, trapping him in a capture container.
Yuna arrives home, but Stitch is nowhere to be found. She wanders through the city and sees Flute (small and cute again) pulling a wagon with the container with Stitch wrapped up. When asked where he's going, he says "Well, I'm going to see Ham-" but cuts himself off before he says Hamsterviel's name. He covers it up, saying he's going to get a ham sandwich at a deli. Yuna remembers what Stitch said about him being a liar, and as Flute turns a corner, the container falls off, and the wrapping becomes undone, showing a small part of Stitch. Yuna sees this and realizes the truth, but Flute hurriedly puts the container back and runs away. Yuna calls Jumba and asks him if he found out where he recognized Flute from. Juma says that he has while looking at a screen showing Flute's original monster form.
At Gantu's bunker, he puts Stitch in the transportation device while Reuben and Flute watch. There's a knock at the door; Flute answers and comes face to face with Yuna, Jumba, and Pleakley. Flute attempts to feign innocence again, but Jumba says that they know he's actually Experiment 145. Flute chuckles and suddenly becomes a lot bulkier; two additional arms also pop out, the right one bearing the transmutant H mark. He tells them that Stitch is about to be sent into space, causing them to run inside, pushing him out of the way. Fortunately, the machine was broken. Jumba points a blaster at them and Yuna gets Stitch out. Flute jumps at them and destroys the machine. Jumba fires his blaster at him, but it has no effect. Flute tackles Yuna, but she lets Stitch out of the container. Stitch throws Flute through the wall and chases him.
Jessica, Toriko, and Makiko see the mutated Flute and Stitch chasing him. Flute hides in a tree, jumps down onto Stitch, and throws him through a park wall. He gloats that he's unstoppable, but hears disapproving voices and turns to see a whole crowd of once-adoring fans around him. He tries to convince them that he's not evil, but it doesn't work and their disapproval drains his power and turns him back to normal. Defeated, he tries to tell Stitch that he just wanted to know what it was like to be cute instead of an ugly monster, but Jumba tells him to give it a rest already and puts him in a container.
In the floating base, Delia punishes Hamsterviel's failed scheme of making an experiment look sweet by making him look sweet...by smearing coconut-creme icing all over his face.
To apologize to Stitch for not believing him about Flute, Yuna makes six plates of sushi just for him, which he loves. Yuna then tells Stitch that he eats too much, has zero musical skill, smells kind of bad, and causes trouble everywhere he goes...but he's her best friend!