"Watashi" is gender neutral, and either gender can use it. Foreigners, unless they're pretty confident with language and familiar with Japanese, they're talking to, are usually encouraged to use "watashi".
Watashi: 私 (わたし)
In formal situations, it's the safest choice and doesn't carry any gender nuances. In casual speech, however, わたし can give off a “feminine” feel, and it's typically only used by women. When men use it in a casual context — when talking with a friend or family — it'll sound stiff.
This talk demonstrates how speakers linguistically enact innovative identities despite the restrictions imposed by linguistic norms. Japanese first-person pronouns are gendered; watashi for girls and women (and men in formal situations) and boku for boys and men.
Watashi: Very formal It is used by men and women, In the business environment, in the public address and in daily conversation among adults who are not close friends. Both phases mean, "I" only for male. Conversational Japanese. Ore: usually the male who is over 16 to 18 years old call himself.
僕 (boku) carries a masculine impression; it is typically used by males, especially those in their youth.
In formal or polite contexts, “watashi” is gender neutral. However, when it's used in informal or casual contexts, it is usually perceived as feminine.
?? Senpai (先輩、せんぱい)
As with "Sensei" is used interchangeably by sex, and does not necessarily follows the name. You might find it transcribed as "sempai". Its opposite is "Kohai/kouhai" but it is rarely used when talking to someone.
BOKU is for males and KIMI is referring to a female and is equivalent to ANATA. However in some songs the girls use BOKU as you mentioned.
The Feminine Pronoun: 私 (あたし) — Atashi
Although watashi is accepting of everyone, the pronoun atashi is primarily used by women. Japanese doesn't have helpful articles like the Spanish “el” or “la,” which can make learning about pronouns a little tricky.
"Watashi" is a pronoun in Japanese that means "I" or "me." It is typically used by people of all genders.
In Japanese, "~ san （～さん）" is a title of respect added to a name. It can be used with both male and female names, and with either surnames or given names. It can also be attached to the name of occupations and titles.
"Watashi wa" (私は) in Japanese means "I am". Watashi Wa.
The verb is aisuru (愛する), to love. And to say I love you in Japanese, you would say aishiteru (愛してる). Aishiteru is a gender-neutral term. So, to say I love you to a man, you'd say aishiteru yo, and to a woman, aishiteru wa.
One of the first person pronouns 俺（おれ）"oré" was used by men and women, educated and uneducated as well in Edo era (1603-1867). Now it's used only by men when he is with his family or friends.
In Japanese conversation – this can be seen as unnatural or weird. In English – the equivalent might be saying something like “I am the one who is Sam.” Versus “I am sam”. So in this case – we can omit the (Watashi wa) and simply say – “Sam desu.”
Boku is, on one level, very simple. It just means “I” or “me.” However, nothing is ever quite so simple in the Japanese language. In Japanese, there are multiple different words for the simple pronoun referring to oneself.
Boku is somewhat more casual and is only used by males, mainly non-teenagers (so kids, and adults). Ore is considered "vulgar" (vulgar in the sense of popular slang), and is extremely common among teenage boys.
ん (romaji n) The hiragana syllable ん (n). Its equivalent in katakana is ン (n). It is the forty-eighth syllable in the gojūon order.
Otokonoko (男の娘, "male daughter" or "male girl", also pronounced as otoko no musume) is a Japanese term for men who have a culturally feminine gender expression. This includes amongst others males with feminine appearances, or those cross-dressing.
A female partner (girlfriend) is called “kanojo,” and a male partner (boyfriend), “kareshi.” Ex-partners are called “moto kare” (ex-boyfriend) or “moto kano” (ex-girlfriend). “Kare” (boyfriend) comes from the word for the male third person “kare” (he), and “kano” (girlfriend) from the female “kanojo.”
Reading: Kareshi / kanojo ni natte hoshii desu. Meaning: I want you to be my boyfriend / girlfriend. Informal / Casual Form: 彼氏 / 彼女になってほしいな。 (Kareshi / kanojo ni natte hoshii na.)
Honorifics are gender neutral, but some are used more for one gender than the other. Kun, for example, is used more for males while chan is for females.
Senpai is often used in the English in the expression “notice me senpai” and its variants. Originally this was used in the context of a person hoping a crush or someone they admire will pay attention to them, but it has more broadly been used online in reference to famous people acknowledging a fan's existence.
Senpai is a Japanese phrase that can be used to refer to someone you look up to or someone you like. In many animes girls use the word 'senpai' when talking about someone who is older than them or just someone who you like/ love.