妹 - younger sister. When referring to sisters, you would write it out as "~姉妹".
彼女 - she/her. It can also refer to one's girlfriend.
責任感 - a sense of responsibility. "責任" means "responsible" and "~感" means "a sense of". "~感" can be mixed with wide varieties of kanji to express feeling or emotion.
強く - to be strong. Note that the kanji can have a different meaning when used with a different kana suffixes (送り仮名 okurigana). The plain adjective is 強い .
成績 - grades/performance/result.
優等生 - an honor-roll student. The antonym for "優等生" is "劣等生" .
電子工作 - electronics or electric work. "電子" means electrical and "工作" means arts & crafts.
得意 - to be skilled or gifted in a particular field. "得意" is also used when someone talks or behaves in a prideful manner. It may also refer to customers or clients when used with "お~" or "~先".
だが - a sentence connector that means "but", "however" or "nevertheless".
的外れ - to be off track/point. "的" means "target" and "外れ" means to "miss".
行動 - action/behavior.
多い - to be in abundance.
いたずらっ子 - a prankster. When you break it down, "いたずら" means "prank/mischief" and "~子" means child. "~子" can also be mixed with different kanji to create a new meaning.
~でもある - to also be something.
Keywords from previous MoeManga posts are as follows.