Tsutsumi has popped up as part of a small sentence in many of the entries I've contributed to "Kayo Kyoku Plus", but I've yet to write anything about the composer himself. So, to give a quick rundown, he was born as Eikichi Watanabe（渡辺栄吉）in 1940 in Tokyo, and began playing the piano at a very young age. His entire education was connected through the Aoyama Gakuin University schools and as a university student, he started getting into jazz. Once he graduated in 1963, he started work at Nihon Gramophone which is now known as Universal Music. There, he was placed in charge of the Western music, and at the suggestion of his sempai at Aoyama Gakuin, lyricist Jun Hashimoto（橋本淳）, he began writing music under his nom de plume of Kyohei Tsutsumi.
Now, according to his official profile on J-Wiki (where I'm getting the information on Tsutsumi), his chosen profession of composer started officially in 1967 but he had already been creating songs before then. Case in point: according to another article for teen pop singer Hiroshi Mochizuki（望月浩）, there was a cute teen song written by Hashimoto and composed by Tsutsumi, titled "Kiiroi Lemon"（黄色いレモン...Yellow Lemon）which had been sung by Mochizuki as his 7th single in September 1966. The song got me thinking of folks like Neil Sedaka and Paul Anka, and in fact, a member, Gus Backus, of the doo-wop group The Del-Vikings did his own cover of it under the title of "For I Smile 'Cause I Think Of You". Unfortunately, the YouTube video for Mochizuki's original was taken down but I was able to find another cover version by Little Patty at the above link.
Most of the entries here have already been covered, so I will provide the links to their articles without having to repeat myself. Moreover, since the breadth and depth of his contributions to Japanese pop music have been so huge as the most prolific (yep, I'm using that term again) composer, I'm going to go with J-Wiki's categorization of his biggest hits by decade.
His first breakthrough came in 1968 with the aforementioned Ayumi Ishida（いしだあゆみ）hit, "Blue Light Yokohama"（ブルー・ライト・ヨコハマ）(although according to J-Wiki, his first minor hit came with "Barairo no Kumo"（バラ色の雲...Rose Clouds）for the Group Sounds band Village Singers). I'm never going to be able to set foot in the Minato Mirai 21 neighbourhood again without hearing that horn which starts off the song.
Apparently in the 80s, his top 3 songs then all were sung by over-caffeinated aidoru Masahiko Kondo（近藤真彦）, with the top rung being occupied by Tsutsumi creation "Sneaker Blues"（スニーカーぶるーす）. I would guess that Matchy sends New Year's cards annually to the composer after that feat.
This is the one song that I hadn't written about previously. Tsutsumi's most successful hit in the 90s was "Ningyo"（人魚...Mermaid）by NOKKO. For the lead singer of 80s pop/rock band Rebecca, this was her 1st single following the first breakup of the band in 1991. Released in March 1994, the song had me first thinking whether Tsutsumi and NOKKO (who wrote the lyrics) were going for an ethereal ballad reminiscent of something that would have been made almost a century back on the Vaudeville stage. "Ningyo" was used as the theme song for a Fuji-TV drama series "Toki wo Kakeru Shojo"（時をかける少女...The Girl Who Leapt Through Time）. Not sure if this had anything to do with the 1983 movie starring Tomoyo Harada（原田智世）, but it did have future superstar Namie Amuro（安室奈美恵） in a supporting role. In any case, "Ningyo" hit No. 2 on the Oricon weeklies and became the 32nd-ranked song of the year.
Yup, if you're excited about Bullet Trains whizzing past you at hundreds of kilometres per hour, you could do worse than having Tsutsumi's "AMBITIOUS JAPAN" soaring through your ears. The song by TOKIO was his biggest hit in the first decade of the 21st century.
We're still not quite halfway through the 21st-century teens yet but it looks like a Kyohei Tsutsumi song has also been making the rounds. Mind you, "Sentimental Journey"（センチメンタル・ジャーニー）was originally sung back in 1981 by aidoru Iyo Matsumoto（松本伊代）, but it's been the popular Tsutsumi tune this decade. The cover version is sung by Ami Maejima（前島亜美）from the aidoru group SUPER☆GiRLS which formed in 2010, and the above video is so pink and sweet, I have most likely gained a couple of kilograms in the last few minutes. Even my usual Xmas binging isn't quite that successful.
For comparisons, have a look at the original performance and the article on the Matsumoto hit.
Currently at the age of 74, I'm not sure whether his pace is quite the same as it used to be, but I wonder if he may just end up whipping up the official song for the Tokyo Olympics in 2020. However, for those who want to take a listen to a retrospective of his long history, there is his "Hitstory Ultimate Collection 1967 - 1997".
For Marcos V, alina and JTM, feel free to create your own retrospectives on a favourite songwriter.