watakushi ワタクシ

In Japanese, watakushi ワタクシ is a very formal first person pronoun.

It's also spelled わたくし, and spelled with kanji as 私, but this spelling is sometimes avoided due to a more common word being spelled with the same kanji: watashi 私.

政宗様はご存じないはずですわわたくしが勝手にご恩を感じているだけですから･･････

Manga: Masamune-kun no Revenge, 政宗くんのリベンジ (Chapter 9, 貞淑なる爆弾)

Nuances

Nuances

The pronoun watakushi is generally considered to be very rigid and often unnecessary.

In most formal situations, business situations, etc., the pronoun watashi suffices, so there's no need to use watakushi unless you want to sound extra formal.

One case in which watakushi is typically used is in job interviews, to present yourself well. Alternatively, jibun 自分 may be used instead.^{(detail.chiebukuro.yahoo.co.jp)}

The word watashi actually originates in watakushi, hence why they can both be written as 私. The word atashi あたし is also related, and it has a more rigid variant: atakushi あたくし.

One difference between the two words is that watakushi is more humble than watashi, so it's sometimes used with the condescending pluralizing suffix ~domo ～共.

watashi-tachi
私達
We.
watakushi-domo
私共
(same meaning, but humble.)

Another condescending suffix used specifically with watakushi to sound extra humble is ~me ～め.

watakushi-me
私め
I.

In anime, characters that use watakushi tend to be either very rich or working for very rich people. These include:

Servants, maids, butlers.
Businessmen from large corporations, not your average salaryman.
Rich girl (ojousama お嬢様) characters, who also often ending sentences with ~desu wa ～ですわ, and use the polite ~sama ～様 honorific suffix instead of the usual ~san ～さん.
- Rich boy (bocchan 坊っちゃん) characters tend to use boku 僕 instead.

One example of an ojousama using watakushi:

Manga: Masamune-kun no Revenge, 政宗くんのリベンジ (Chapter 9, 貞淑なる爆弾)

Context: Fujinomiya Neko 藤ノ宮寧子 changed schools to be in the same class as Makabe Masamune 真壁政宗, when his classmates tell him about it, he seems surprised, and Neko explains:
Masamune-sama wa {gozonjinai} hazu desu wa
政宗様はご存じないはずですわ
Masamune-sama should {not know about [it]}. (literally.)
I don't expect Masamune-sama to know about it.
{watakushi ga {katte ni} go-on wo kanjite-iru} dake desu kara......
わたくしが勝手にご恩を感じているだけですから･･････
Because [it] is just [that] {I felt [his] favor on my own}. (literally.)
I just grew feelings for him on my own.
- In other words, it's unrequited love: she started liking him by herself, and he was never informed about it.