Mayuka is only seen in the second Tenchi Muyo movie, Manatsu no Eve. She first appears as Tenchi's Daughter. You're probably thinking "What???" or "NO THAT CAN'T BE!!!" but its true.
Yuzuha, a demon, was watching Tenchi and the gang having a Christmas party. She notices that Tenchi seems to be the "Prince" and takes a bit of his hair. She pulls a little of her hair out and races down to a jar of green liquid in it. Then she puts the hair in it. She creates a stone that becomes Mayuka and sends her to Tenchi's house to kill him.
Mayuka appears on the shrine steps behind Tenchi as Tenchi was walking back to the house. He stops suddenly when he hears a sweet little voice call out "Papa!". Tenchi brings her to his house, and everyone is shocked. Ryoko freaks out a couple of times (actually more than a couple), and causes a fight between the two. Mayuka never tells them who she is, because, really, she doesn't know. All she really knows is that Tenchi is her Papa, she knows nothing of her mother. Soon Mayuka makes really good friends with everyone, except Ryoko.
Mayuka has no knowledge about why she's there, all she knows is that she has came to see her Papa. But, Yuzuha uses her by a Music box Mayuka brings for Tenchi. When the music box opens, Mayuka becomes possessed and tries to seduce Tenchi and her pupils become really small. As she holds him close, a light forms around them and they start to float. But as they float in the air, someone stops them, and they fall back. Mayuka changes back to normal, and has no clue what happened.
Mayuka and all the other girls (except Ryoko) were in the bedroom in their pajamas, and Mayuka asked Sasami what Christmas is. She tells her that it's a lot like Startica, but its in the winter. They pull out a picture album and talks about the pictures and has fun. Mayuka said she wants to celebrate Christmas too, so Aeka tells her that she can be in their family and celebrate Christmas with all of them, if she was a good girl and wait for winter to come. Mayuka was so happy! Meanwhile, Yuzuha was watching them, and was getting really pissed.
They went to sleep, and as they were sleeping Mayuka's music box opens and a hologram image of Startica appears. The image changes to Yuzuha and asked her who told her to be friendly to them. She tells her to bring the Prince (Tenchi) to her, or she will destroy her. Mayuka wakes up scared and walks to Tenchi's room with her music box. While he's sleeping she lays the box down and says "Papa, I'm scared. Papa". The music box opens and tells her to get to it. Mayuka becomes possessed, and starts to seduce Tenchi. As she does, a bright light surrounds them, and they are lifted up. Yosho and Washu comes in, and Yosho tries to attack Yuzuha, but she disappears. Then Ryoko appears. The light fades, and Tenchi and Mayuka fall back on the bed. Mayuka opens her eyes and gets very scared. Ryoko grabs her and holds her up in the air, asking what that thing was. Yosho tells her to stop, because she doesn't know anything about it. Tenchi wakes up and sees Ryoko holding Mayuka in the air, and she drops her. Mayuka tells Tenchi that she really doesn't know anything. Ryoko asked her what she was going to do with Tenchi, and Mayuka runs away telling them that she doesn't know. Tenchi tries to run after her, but Ryoko grabs him because she doesn't want him to go and get hurt.
Tenchi and the gang all go outside, Yosho explains that its not really Mayuka who is doing this, Yuzuha is possessing her. Ryoko says that she is still the enemy, but Sasami says that they are cruel and runs off to find Mayuka. Mayuka runs through the woods, and suddenly, she gets flashbacks of what she has done to Tenchi. She gets so scared she falls to the ground and starts to cry. The music box opens up and Yuzuha tells her that she is disobeying her, so she will make it where Mayuka will never do it again. The stone Mayuka was created from appears on her chest and Mayuka screams in pain as bolts of energy surrounds her body. Sasami comes and runs to Mayuka, but possessed Mayuka gets up and grabs Sasami. Aeka runs to them but is attacked by Yuzuha and she falls to the ground. Tenchi and Ryoko runs to Aeka, and Yuzuha, Mayuka, and Sasami disappears.
Washu creates a vortex to Yuzuha's lair, and Tenchi and Ryoko jumps in it. They appear in Yuzuha's room, with a bunch of toys. The toys start to laugh, and a bear introduces themselves. As he introduces themselves, all the toys have knives that pop out of them, and they attack them. After running for a while, a hole appears below them, and they fall down to a room with Mayuka in a Christmas outfit, she says "Merry Christmas". Sasami is trapped by roots on a tree. Mayuka and Ryoko fight, and Ryoko hits her with blasts of energy, and Mayuka's clothes vanish. As Mayuka falls to the ground, she changes her form, she grows fur. She attacks Ryoko and Tenchi yet again, but this time more powerful. Yuzuha shows up and Mayuka jumps to her side, and kneels beside her. She tells them that she just wants to play with them, and then attacks them. Ribbons fly around everywhere, and wraps around Tenchi and Ryoko. Ryoko gets them free, but more ribbons go after them and they fall into a hole. Luckily they land on something soft, but then it begins to move. They come to realize that it is a huge Teddy Bear. Yuzuha flies around on her flying Yoyo and grabs Tenchi and drops him, but Ryoko flies down to catch him, which gets attacked by the huge bear and Mayuka. The bear hits them down yet another whole, and Ryoko passes out.
Yosho, Aeka, Washu, Keyone, and Mihoshi wait by the portal, and Aeka finds out that Jurai power cannot work down there. She runs to the tree that belongs to her brother (Yosho) to see if she could help them. Meanwhile, Tenchi and Ryoko fall into the Green Liquid that was at the beginning of the movie. Yuzuha flies to them, then Mayuka shows up with Sasami. Yuzuha asks what she should do with Sasami. Tenchi tells Yuzuha that he would stay there if Sasami and Ryoko could be sent back. Mayuka starts to attack Tenchi, digging her claws into him, ripping his shirt off. Mayuka grabs Tenchi by the neck and strangles him in the air. Sasami wakes up and sees Mayuka doing this and yells to her to stop it. Mayuka looks at her, then continues to choke Tenchi. Sasami yells at her asking her what she is doing, and to stop it, he his her Papa. Mayuka looks and her and starts remembering things. She remembers of all the fun she had with them. She looks at Tenchi and screams "PAPA!!" and hugs him. She starts to cry as Tenchi says her name, but an enraged Yuzuha flies towards them, and knocks them apart. She then attacks Mayuka. Mayuka starts to act evil again, and attacks, not Tenchi, but Yuzuha. She says "Yuzuha is Yuzuha, Mayuka is Mayuka! I would prefer Christmas to Startica, because Papa, Sasami, and everyone will be there with me. I'll be a good girl and wait for the winter to come, so that I can celebrate Christmas with all of them!" Yuzuha screams that she is a good for nothing fool, and to die. She attacks Mayuka again. Tenchi, Ryoko, and Sasami run to her as she's laying on the ground.
Mayuka: Papa. I'm
so sorry I did terrible things to you.
Mayuka begins to glow, and disappears with Sasami screaming her name. Tenchi sadly guides Sasami away, and looks down. All that is left is the stone she was made from. Tenchi told Yuzuha that she just killed her own half, but Yuzuha didn't care, and started to change her form. She grows wings and fur, and attacks them. Ryoko grabs Tenchi and Sasami and flies away, but Yuzuha grabs Tenchi and Ryoko as she grows huge. Yuzuha says "Look at me. Can you say you love me like this? I wish I killed your grandfather back then." Suddenly light appears and Yuzuha covers her eyes. Aeka appears in the light and yells out "Tenchi-sama! Behold! Light prevails even in the world of darkness!" and throws a stem from the tree to Tenchi. He then changes appearance and holds the Light Hawk Sword. He races toward Yuzuha, and slices her through. It shows a flash back of her past, when she met Yosho, and she fades as she says "Imagine the darkness in love with the light... "
They awaken in the forest with the rest of the gang there. Tenchi gets the stone that Mayuka was made from out of his pocket and shows his grandfather. They go back to the house, and they talk about how Mayuka is not here with them, and how much she wanted to spend Christmas with them. Even Ryoko cries, which catches Washu's attention, but Ryoko denies it. Washu tells Tenchi to give her the stone, and she says to them all "Listen, everyone. Since this is a matter of great importance I'm going to ask you only once. What shall I do with this? I'll do what everyone wishes. If there's one among you who doesn't wish it to happen I will not do it." Everyone looks at Ryoko, and Ryoko says "I wish I could see her again." Everyone is so happy, then Ryoko tells them that she will set her straight this time. The credits come up, and as the credits roll, it shows the stone, growing into a fetus.
The credits features pictures of Tenchi and the gang having Christmas that year, with baby Mayuka.