永遠 - for an eternity.
ぴちぴち - young and lively.
高校2年生 - second grade of high school or 11th grade.
人知れず - to do something in secret and unnoticed by many or all.
戦う - to battle or fight.
戦士 - a soldier or a warrior.
~こと~ - means "also known as".
一員 - a member of a group or an organization.
元気 - cheerful and energetic.
優しい - kind and generous.
しっかり者 - a responsible person.
みんなに好かれる - to be liked by all.
刀 - a Japanese sword.
よく - often.
振り回している - to swing something around. The plain verb is "振り回す" .
This is a compound word made up by the following two words:
シャイ - to be shy. Words like these are called "和製英語" - a Japanese-English word derived from English.
芯はしっかりしている - to be determined or have a strong heart. "芯" usually means "core" or "center" but depending on the situation, it can also refer to the person's "heart/will".
おっぱい - women's chest. A premature oppai on the other hand is called "ちっぱい" .
大きい - large.
いじられる - to be made fun of. The plain verb is いじる .
There's a thin line between "いじられる" and "いじめられる" which means "to be bullied". If it's something you can laugh it off then it's an "いじり". However when people take it too far, then it's an "いじめ".