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2000 Maki Goto
Maki Goto is known to have gone out with some Johnny's Jrs during their early years. How she managed it despite being an idol herself is beyond me. Both parties denied the relationship, but the tabloids talk about it like it's a fact. Maybe it is. So one story I heard was that a crazy Hello Project fan once threw water (other sources said it was hydrochloric acid) at Yamapi, because of...jealousy I guess.
2008-2009 Abiru Yuu
At the beginning of September 2008, Yamashita Tomohisa was seen with Abiru Yuu at Kyoto’s Jinushi shrine and spent the day as sweet as any normal couple would be This resulted to a lot of fans screaming murder. It's not really that surprising since both attended Horikoshi Gakuen where they started off as senpai and kouhai. These years are important since these was when the first foundations of their relationship came to be and why YamaPi and Koki Tanaka hated each other.
According to Cyzo, they like different types of girls, but in the end both guys fell for Abiru Yuu. Koki and Abiru used to be in a relationship for a while, but eventually broke up. Now single, Abiru became close to YamaPi this time. When Koki discovered this, he tried to get back with Abiru. This lead to a dispute between her and Yamashita. Soon they decided to have a fresh start and start all over again. Yamapi's close friend and then class leader, Shirota Yu got pissed and bullied Koki in class up until graduation. I don't know their current relationship with each other these days but by 2008 YamaPi and Abiru finally ended as couples.
During the time YamaPi was still working on Kurosagi, it was rumored that they are wearing couple necklaces also that they've been seen eating diner together or driving together. However they eventually broke up by January.
What happened? Some sources say that the major reason of the breakup is that Yamashita found out Abiru had affairs with some other guys. Another possible reason for their break up was the jimusho not being too fond of Yuu's image. She had been Koki Tanaka's girlfriend in the past and one time she admitted on a TV show how she did a bit of shoplifting as a girl. All these prompted Johnny's to speculate that she may be ruining YamaPi's image.
Another BUBKA source shared the following:
“It happened in March at an after party. They met at a private movie screening. But the girl he called out to meet was a half-Japanese model that he liked who is Abiru’s friend, Kagami. After that, they went to a club and Yamashita started to develop some feelings for her,”2009 Kagami Seira
According to BUBKA, they spoke to a staffer who has proof, a photo, that the two have been living together . Hell, they even have their own namesquish, "Serapi" how cute is that? According to an insider, "Yamashita loved this name so much and would say it any chance he got; he wanted show off how in love they were." One time, Yamapi was so busy they couldn't really go out on dates anymore, so as a way of making it up to her, he had 100 pcs of t-shirts made. The design was yellow and had "Serapi~" spelled out with pink hands.
However, Kagami was pissed instead. "She was fed up with his childishness." Tokyo Sports reported. This was the final straw and Kagami was the one who put an end to their relationship. I am both impressed by the T shirts and the We-Are-Not-In-Highschool-Anymore scenario.
2009- 2011 Keiko Kitagawa
|But this could be anyone honestly|
It all began with a purikura that became viral that way back in 2006. Not surprisingly, a lot of fans posted rude comments on Keiko's blog, promting the actress to deny any kind of relationship with Yamapi, stating they were only classmates and he is only an acquaintance. It could be true, since she's rumored to be with Akanishi Jin/Shirota Yuu during that time. It's also noteworthy to remember that both studied commerce at Meiji University. Looks like their friendship began way back.
|Pi and Keiko talking in class|
Everything seems to be going well...until...
2011 French woman
January 23rd. A cold rainy night in Nishiazabu. Yamapi appeared in a Japanese restaurant, known to host the Prime Minister and other VIP visitors.
"It looked like a fun double date with two white girls and another good-looking Japanese guy. The atmosphere was relaxed. From what I overheard of their conversations, it was in English. I thought 'Yamashita-kun can speak English~,' it surprised me. When we were entering the restaurant, they were hugging at the entrance. With the slim foreign girl, it looked like a scene right out of a drama. In the restaurant, they didn't care who were looking and sat near the entrance. The girl was talking to him and I heard her asking 'You are okay?' and also saw him affectionately brushing her hair to whisper in her ear." (A customer)
A friend of Yamashita Tomohisa shared that the woman turns out to be Yamashita Rina’s English teacher. He is living together with his family at a large apartment and thus it was inevitable for him to get to know her.
“She was supposed to teach Rina English, but he often joined their private lessons in order to improve his English as well. This is the main reason why he has gotten so much better within just half a year. At the same time she must have become more than just a teacher to him…” 
“Yamashita said that he could relax around her because she didn’t see him as a celebrity. He’s been hoping to work overseas for years, and I think she had a good effect on him because they talked to one another in English. Even if he had work from early in the morning, he’d make time to see her for meals afterwards,” said one source.Their relationship didn't last long
“They broke up ages ago. Around the beginning of last ear there were a number of sightings of the two of them on dates and there was talk that they were kind of living together, but I heard they broke up around the end of the year. I think the cultural differences were too much for him to handle. Western culture has influenced his lifestyle a lot lately, but there might’ve been some things about how she saw the world that he just couldn’t understand,”2012 Restaurant Worker
Things with the gaijin didn't work out I suppose since out of the blue, news of Yamapi having a new date surfaced. That time he’s already introduced her to his close friends, and they all went for a holiday in Okinawa during the spring break. He didn't want to attract attention so they went to a smaller island away from the main island.
“B-san works at a restaurant in Tokyo. But because his agency told him off for being too careless while dating his last girlfriend, he’s been really careful when seeing his new girlfriend. It’s a big change, because he used to tell everyone that he didn’t care whether people were looking at him,” said one source who knows Yamashita.2013 Oshima Yuko
Oshima Yuko dating Yamashita Tomohisa rumors was first fueled when the two were spotted in New York. On twitter, pictures of Yamapi arriving at Narita Airport last Jan 5th has become a hot topic. Nothing suspicious about that. However, a certain netizen also claimed to have seen Yamapi and Oshima in New York and have gone shopping in Hermes.
There were photos too to 'prove' this sighting, but it's too shitty to even decipher a face. Nonetheless, tongues started whispering that they are now dating in secret. It is also interesting to take note that certain eyewitness also tweeted that they saw the two of them together, describing Yuko as “thin Asian and stylish woman”.
SOURCES  zurui-koi.livejournal.com/13959.html?thread=458119