Senpai (The Original Protagonist)

Biography
Senpai is your everyday Japanese highschooler whom has just recently moved into the Town. His family consists of both his parents and his oldest sister the Detective. The relocation in fact, was actually due to his sister taking a high profile job in the Town. Perhaps the decision to move was also helped due to already having close family living there. Its said by the Little Sister herself that she's being babysat by cousin the Detective in Episode 41.

Personality
As for Senpai's personality, it can be said he's someone to depend on. As he's noted for helping out Hikari with a book and another student with a bloody nose. Though most of the time he's quite reserved, he can also be quite direct. In Episode 06 he stated he would rather spend the rest of the day with Okiyaki, much to her embarrassment. Senpai also appears to be serious about his studies.

003 Diary Of A Yandere Prologue
[Monday, June 5th to Friday July, 7th 2017]

On [Monday, June 5th] Hikari writes her first entry about Senpai the new transfer student. Later on in the day, she sees him shyly spending his time away in the bookstore. They meet again at school on [Thursday, June 8th], when Senpai helps a fellow classmate to the nurse's office. From then on Senpai doesn't have anymore contact with Hikari except for the after-school stalking beginning on [Monday, June 19th]. As of [Thursday, June 29th] Senpai had already become friends with Suzume Okiyaki. Finally on [Friday July, 7th], Senpai is seen going home with Okiyaki before he is kidnapped.

006 Date With A Tsundere
[Takes place on the weekend sometime from the beginning and end of "Diary of A Yandere Prologue". Most likely occurs sometime between Saturday, June 24th and Sunday, June 25th 2017 due to the rain.]

Senpai meets with Suzume Okiyaki at her favorite cafe for a date. When she receives a call to meetup with her friends Senpai declines their offer. Instead he suggests they continue their date for the rest of the day. Okiyaki barely keeps her tears in before storming off.

051 Big Sister Helps You Study Roleplay
[Sometime after "Date With A Tsundere". Late at night in Senpai's room. Likely to have occurred on Thursday July 6th 2017.]

Senpai is in the midst of studying when his sister walks in for a serious talk. Brushing off her warnings about the Town he instead ask for help understanding Okiyaki's strange behavior. She teaches him Okiyaki is only over-aggressive because Senpai is too dense not to ask her out first.

024 Tsundere Love Confession
[05:07 PM Friday July, 7th 2017]

After-school Senpai decides to meetup with Okiyaki in the courtyard. Finding that they share mutual feelings both decide to date. Holding hands they begin walking home only to encounter an enraged Hikari.

001 A Yandere Kohai Kidnapping
[Saturday July, 8th 2017]

Waking up at Hikari's house, Senpai finds out he's tied down to a chair. He remains calm even playing into Hikari's confession until its made clear he will never be freed. After shouting out for help his mouth is duct taped and left to think over his behavior.

041 Detective Interrogation
[Somewhere between July 8th and the end of August 2017]

Senpai is briefly mentioned by his cousin the Little Sister. She explains to the Butler that her cousin, the Detective, is babysitting her. She mentions that the Detective has been on edge ever since Senpai went missing.

007 Breakfast In Bed A Yandere Kohai Kidnapping Part Two
[Late Morning, August 2017]

Waking up in Hikari's bed, Senpai is forced to listen to her rambled about brunch. He eventually glances on the hammer that was used to break his legs last night when he resisted again against Hikari. To which she immediately downplays before placing it away before leaving to go cook. When Hikari later tries to get Senpai to eat he begrudgingly refuses. Only after she threatens to break all his fingers does Senpai begin to eat.

016 Anniversary Dinner A Yandere Kohai Kidnapping Part Three
[September 2017]

On the two-month anniversary of their relationship, the couple begins their dinner before someone comes knocking on the door. Taking the opportunity to test Senpai's love she kidnaps the intruder and gives Senpai a knife to prove his sincerity. Breaking down before on the ultimatum, he decides to kill the student much to Hikari's delight.

030 The Necromancer
[All-Hallows-Eve, October 31st 2017]

Suzume Okiyaki mentions that she was killed for something as petty as a "boy".

034 END A Yandere Kohai Kidnapping Part Four
[Four? Five? Six Months after July; Presumably sometime in November 2017]

Senpai briefly dreams of when he first met Hikari all the way back then in the bookstore. He wakes up to the present Hikari back in her house after the vacation. It's stated that sometime during their vacation that Senpai was able to strike an agreement to stay with Hikari as long as she stopped with harming people. By this point his mobility is limited to crutches. He becomes excited by Hikari's offer of a dinner party with very special guests. However once its made clear the special guest are instead Hikari's very dead parents he lashes out. When Hikari decides to commit suicide together Senpai briefly attempts to flee until he's knocked out. The next day he wakes up tied down again. Hikari thanks him for waking her up before plunging a knife into his chest and later her own.

035 Literature Club The Raven
[Late November or Early December 2017]

A fellow female classmate notes the danger of staying after school especially after the discovery of Senpai's and Hikari's decomposing bodies.

Detective
He's shares a pretty strong bond with his older sister and feels comfortable enough to ask her about relationship advice.

Tsundere
Though they originally began as friends both secretly held a crush on each other. After some advice from his older sister, Senpai was able to accept her confession.

Senpai is one of the few people the Tsundere feels comfortable enough to convey her "dere dere" side.

Hikari
Initially, Senpai is clueless to who Hikari is as both are separated by grade levels. He simply thinks of her as someone who needs help from time to time.

During captivity he initially decided to play along before realizing he would never leave. Upon every failed attempt of resistance he was met with more and more punishment. Soon after he was crippled Misery-style after a failed escape. Because of this he carries a grudge on Hikari until he's force into complying under force. In time he begins to lose himself playing along with her delusions. After just two months he's even capable of killing a fellow student just to potentially escape. His dreamy flashback reveals he still has trouble associating the shy Hikari he knew and the insane killer he's stuck with. Despite his rejection Hikari decides to kill him to fulfill her happy ending.

Trivia
As a fan of Higurashi, fallenshadow most likely based Senpai on Keiichi Maebara. Both have recently moved in town, live in a considerably nice house, and easily become popular. However, both are also confronted with increasingly strange behavior by the girls they meet before their bloody ending.