Tsugumi Ohba is the name of the mangaka, along with artist Takeshi Obata, responsible for the Shonen Jump series Death Note. This is, so far, the only manga attributed to this name. I say 'name' rather than 'person' because it is strongly suspected that Tsugumi Ohba is a pseudonym, however the true identity of the Death Note writer is still widely unknown.
To successfully get a manga series printed in the popular Japanese magazine Shonen Jump it is expected that the artist or writer has achieved a handful of competition wins at the very least. They must have a selection of previous work to prove their worth, as the magazine does not tend to publish first efforts. However, Death Note marks the first and only story by writer Tsugumi Ohba. It is also suspicious, to anyone who is aware of the plot, how a writer managed to create a story as rich, well developed and complex on their first attempt.
The profile of the author provided in the manga's tankobon claims that Ohba was born in Tokyo, Japan; hobbies include "collecting teacups" (an odd hobby shared by Philip K. Dick) and developing manga plot lines while holding knees in a chair (similarly, Death Note character L has an identical quirk).
There is speculation that the pen-name is in fact used to hide the identity of a famous mangaka by the name of Hiroshi Gamou. Gamou is the author of Jump series Tottemo! Luckyman, a much more child-accessible series. It is suggested that he used the pseudonym to separate his past work from the change in tone to the darker, morally ambiguous Death Note. A possible clue to this is the name of the school protagonist Light attends in the first chapter (Gamou Seminar), which is just the sort of inside joke manga writers are often responsible for. Another hypothesis is that the name belongs to a woman, Yuuko Asami, artist of series Wild Half. This claim is supported by some interviews that have surfaced, especially since being made into a live action movie adaptation, where the interviewee is written as a "she". For instance, from an interview on zaobao.com, "Her particular favourite part of the story is, apparently, the part when L reveals his real identity to Light".
It seems Tsugumi Ohba was an interesting pen name to choose too. In Shonen Jump the most popular stories are always printed in the front of the magazine. Series' such as Bleach, One Piece and Naruto occupy the front, while other smaller, less successful manga are put at the back. The last pages of the magazine are referred to as the batsu gumi. When ordering the pseudonym first name, last name, contrary to Japanese standards we get Ohba Tsugumi. Or oh-batsu gumi. Perhaps the author realised that a story this incredible was as likely to not capture the collective mind and fail, as it was to succeed - making Tsugumi Ohba similar to the Alan Smithee of the cinema world? It's also worth considering that all the major characters in Death Note operate behind an alias (Kira, L, Near, etc.) in order to protect themselves. Could it be that the author has considered this aspect of the story imporatant enough to implement on a greater level? If that's the case, to deny recognition as a genius of the genre to magnify the importance of a theme in the work is utmost dedication to art.