堕落 (daraku) is falling, and in this word it is the fall from heaven itself, the character's first sense; it came to mean depravity only because that is where the fall ended. 天使 (tenshi) is heaven's envoy, for two thousand years the emperor's messenger and in modern Japanese the angel.
The four characters are real Japanese, the words the translators used when they brought Milton's Lucifer into the language, and they are also the standard Chinese term and a film title, so a Japanese reader and a Chinese reader take them at once.
The word Japanese says in its own conversation is the short one, 堕天使 (datenshi), the dictionaries' headword for the devil and for Lucifer their chief, the manga and goth word for the beautiful rebel; the design writes the long form, which says the thing whole and in two languages. Fallen Angel is the exact English. A buyer who wants the concept has it here, and one who wants the everyday Japanese word has 堕天使.
堕落天使 is composed of the words 堕落 (read daraku) meaning "fall; falling" and 天使 Angel (tenshi).
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 堕落天使.