Mashisu Makibi first appeared in Episode 16 of the third Tenchi OVA as Misao's assistant on board the Kuramitsu family's newest starship, the Choubimaru, which had the capacity to destroy an entire planet. But Mashisu was not Misao's first choice for second-in-command: he had requested Noike for that post, but at the last second, Mikami had Mashisu fill-in.
Mashisu deeply wishes for Misao not to go through with his terrible plan, which was to use the Choubimaru to rescue Mihoshi and the Jurai Royal Family from Tenchi's clutches, then in a battle with Ryoko, the ship's main battery would 'malfunction' and blow up Earth, killing Tenchi in the process, knowing that the residents of Earth were liable to go into a panic seeing a spaceship the size of Earth's moon approaching the planet (in actuality, the letter which informed Misao of Mihoshi's plight was altered by Z, in the hopes this incident would occur). Mashisu tried to reason with Misao to modify his plans, if not for himself, but to spare the pain of having Mihoshi resent Misao because of Tenchi's death. She even went as far as spanking Misao, mostly to spare him the embarrassment of being seen by his crew with a black eye.
Unable to dissuade Misao from his plans, Mashisu secretly came up with an alternate plan by framing Ryoko for the destruction of the Choubimaru. What it involved was that she, along with her subordinates Baguma, Fujimasa, and Sorunaru, would cause the ship's destruction by provoking Ryoko (using some minor offenses still on her record as a justification; her major crimes were deleted) to attack it. During the battle, the ship's core would 'accidentally' overload and explode. This way, she could avoid having to destroy the Earth, not to mention avoid hurting both Misao and Mihoshi over Tenchi's death. Baguma, Fujimasa and Sorunaru were then dispatched to handle Ryoko, Ayeka, and Washu, while Mashisu would go after Sasami.
A plan gone wrong
However for the four, Ryoko, Ayeka (with Ryo-Ohki’s help) and Washu were easily able to handle Fujimasa, Sorunaru and Baguma respectively, while Sasami easily defeated Mashisu with combat and evasion skills that Mashisu was ill-equipped to handle, and helped her notice that she is in love with Misao. The group was then taken to on board Tsunami-fune, which Seto had borrowed for the trip to Earth. It was on board Tsunami-fune that Seto treated the five to an impromptu party, which involved plenty of the rare libation Shinju sake
However, when Ryoko began attacking the Choubimaru in a drunken rage (she had consumed an entire barrel of Earth sake after her battle with Fujimasa), Mashisu became so concerned about Misao's safety that she rushed back to his side. Mashisu confronted Misao on the Choubimaru's bridge, and was able to help him come to his senses and order the crew to abandon ship. However, it took a little longer for Mashisu to persuade Misao to join the crew... which resulted in Misao trying to shield Mashisu from an explosion.
Apparently injured fatally by the blast, Mashisu helped Misao remember the day they had first met and she fainted admitting that she is in love with Misao. On that day, Misao had accidentally hit Mashisu on the head with a tennis ball. Although Misao had apologized for the incident, Mashisu was so enraptured with the young boy she didn't hear the apology, and gave Misao a vicious headlock as a result. But afterward, Misao had invited Mashisu to lunch with his family, where she met Mihoshi. She also made mention of an unknown incident (possibly the one that caused Mihoshi's downfall within the Galaxy Police) which caused Misao to mature.
However Mashisu's injuries were actually an illusion created by Washu on Tsunami-fune, to help Misao understand the importance of who he REALLY should protect. Mashisu, as well as Misao, were surprised to find that the 'explosion' was actually a transport beam that teleported the two onboard Tsunami-fune, and Mashisu was embarrassed that EVERYONE had heard them and her confession to Misao.
A future with the Kuramitsu family
After a modified resolution to the Z incident, Mashisu was punished by Mikami with six months of toilet cleaning in the G.P. headquarters for her part in the Choubimaru incident. When Misao wanted to have Mashisu with him for his portion of the punishment (ten years working in one of the family's ventures, however only one's spouse is allowed to go to the family venture), she was dismissed by Mikami’s brother, now FORMER GP marshal Minami, as "unworthy". However, Seto, who was also present, offered to adopt Mashisu so that she could marry Misao. This part was confirmed when Misao and Mihoshi's mother Mitoto (mop in hand) showed up and informed everyone that she got the Kuramitsu Family's counsel to accept the marriage - which was promptly broadcast all over the galaxy.
Mashisu also appears in GXP episode 15. After her punishment for the Choubimaru incident had ended, she married Misao and became Mashisu Kuramitsu. Apparently as a wedding present, Mashisu was given command of the Choubimaru, with Misao going to the Galaxy Army. Her hair was even longer by this time. She was able to help out when Fuku had taken command of the Kamidake II - with Seina on board - when she ran away to Earth - but with a little persuasion from Amane first, who was there with Kiriko and Ryoko. Mashisu was enraged when the Kamidake II was attacked by pirates of the Daluma guild, frightening Fuku to tears. Mashisu, along with help from Fuku's big sister Ryo-Ohki, chased off the pirates.