ドラえもん:Doraemon is a popular manga in Japan. It is interesting to analyze because the "Dora" part of "Doraemon" is written in katakana, whereas the "emon" is written in hiragana. The manga is about a robotic cat named Doraemon who is from the future and who befriends a boy named Nobita kun. It chronicles their adventures together. Doraemon's favorite food is Dorayaki; it's interesting to note that the ドラ in ドラえもん is similar to Dorayaki. However, Dorayaki can be written in hiragana, kanji, and katakana. "Dora" means "gong" in Japanese, so the "Dora" in Doraemon could be written in katakana because it's an onomatopoeia. Also, "emon" was a popular ending for male names in Japan, so that could be why it's written in hiragana.
コーヒー:This word is a loanword from the English word "coffee." It was also made into a kanji later on. Some foreign words, like this one, are made into kanji called "Ateji." Coffee could be written in ateji because it became a popular word.