「心が変りやすい」といったニュアンスの単語集めました。
capricious [kəpríʃəs] (12) a １．formal 気まぐれな、２．literary (気候などが)突然に変化する
capricious suggests motivation by sudden whim or fancy and stresses unpredictability <an utterly capricious critic>. (Merriam-Webster)
「突然の気まぐれや思い付きによる動機を示唆し、予想外であることを強調」
erratic [irǽtik] (10) a (行動が)一定しない
erratic stresses a capricious and unpredictable wandering or deviating <a friend's suddenly erratic behavior>. (Merriam-Webster)
「気まぐれで予想できないさまよいや逸脱を強調」
fickle [fíkl] (11) a １．気まぐれな、２．(天候などが)突然変わりやすい
fickle suggests unreliability because of perverse changeability and incapacity for steadfastness <performers discover how fickle fans can be>. (Merriam-Webster)
「ひねくれた変わりやすさと確固とできないことにより、信頼できないことを示唆」
flighty [flάɪṭi] (25) a (女性が)移り気な
If you say that someone is flighty, you disapprove of them because they are not very serious or reliable and keep changing from one activity, idea, or partner to another. (disapproval)
Isabelle was a frivolous little fool, vain and flighty. (Collins Cobuild)
「まじめでなかったり、信頼が置けなかったり、行動や考えやパートナーを次から次へと変えたりすることが気に入らないときに使う」
mercurial [mərkjúəriəl] (18) a １．literary 気まぐれな、２．literary 快活な、機転のきく、３．<化学>水銀を含む
mercurial implies a rapid changeability in mood <made anxious by her boss's mercurial temperament>. (Merriam-Webster)
「すばやく気分が変わることを示唆」
Did You Know? (Merriam-Webster)
The Roman god Mercury ("Mercurius" in Latin) was the messenger and herald of the gods and also the god of merchants and thieves. (His counterpart in Greek mythology is Hermes.) He was noted for his eloquence, swiftness, and cunning, and the Romans named what appeared to them to be the fastest-moving planet in his honor. The Latin adjective derived from his name, mercurialis, meaning "of or relating to Mercury," was borrowed into English in the 14th century as "mercurial." Although the adjective initially meant "born under the planet Mercury," it came to mean also "having qualities of eloquence, ingenuity, or thievishness attributed to the god Mercury or the influence of the planet Mercury," and then "unpredictably changeable."
ローマ神マーキュリー(ギリシア神ヘルメス)は、神々の使者、伝令者であるとともに、商業やどろぼうの神。弁が立ち、素早く、ずるがしこい。水星、水銀にその名前はついている。
mercurialの語源は、ラテン語mercurialis「Mercuryに関係した」。最初の意味は「水星のもとに生まれた」、のち「(神マーキュリーのように)雄弁で器用で盗みの才に長けた」、そして「予想できない変化をする」の意味にも使われるように。
余談ですが、マーキュリーが持っている2匹の蛇の巻きついた杖Caduceus（ケーリュケイオン）は、医療のシンボルの1匹の蛇の巻きついた杖Rod of Asclepius(アスクレピオスの杖)とは別物とのこと。
skittish [skítiʃ] (23) a １．(馬などが)驚きやすい、２．気まぐれな、３．(投資家などが)弱気な
1. an animal, especially a horse, that is skittish gets excited or frightened very easily
2. a person who is skittish is not very serious, and their feelings, behaviour, and opinions keep changing
3. if people who buy sharesare skittish, they are nervous and worried about them dropping in value, and might sell the shares that they own because of this SYN jittery:
▪ Some skittish Wall Street investors are staying away from the market. (Longman)
２．「あまり真面目でなく、感情・行動・意見が変り続ける」
volatile [US vɑ́lətl, UN vɑ́lətail] (10) a １．(しばしばけなして)気まぐれな、激しやすい、２．(状況が)一触即発の、３．<物理>揮発性の
1 (often disapproving) (of a person or their moods) changing easily from one mood to another
a highly volatile personality
2 (of a situation) likely to change suddenly; easily becoming dangerous
synonym unstable
a highly volatile situation from which riots might develop
a volatile exchange rate
3 (specialist) (of a substance) that changes easily into a gas
Petrol is a volatile substance. (Oxford Advanced Learner's)
１．「（しばしばけなして）(ひとや気分が)次から次へと変りやすい」
参照
Random House fickle
http://www.dictionary.com/browse/fickle
Merriam-Webster capricious
http://www.merriam-webster.com/dictionary/capricious
Merriam-Webster erratic