I've been staring at this for a while now and can't seem to decide on what is actually happening. The sentence in question is:
さっきから動物に餌を与えるのを楽しんでいるかのようなシンシア
Now does this refer to an actual animal being fed, or is Cynthia feeding someone like they were giving an animal treats?
For context, here is the surrounding exchange:
「ロエル、これ食べる？」
「食べる食べる、ぽりぽり」
さっきから動物に餌を与えるのを楽しんでいるかのようなシンシア。 何でも吸い込んでいくから、面白いのかもしれない。 今ではシンシアの手持ちのお菓子を半分近く消化している。
「はい、あーん」
「もちゃもちゃ」
So is Cynthia feeding animals or ロエル?
She is feeding ロエル as if she were feeding an animal. (まるで)～かのような means "as if ～", which is commonly used to make a metaphoric statement. Lots of examples here.
I guess what's throwing you off is the writing style of this sentence. First of all, of course it's a sentence that fully qualifies as a valid sentence. However, it's not a regular Japanese sentence that ends with a predicate (i.e., a verb or だ/です). This is a sentence which ends with a noun because it only includes one long noun phrase, grammatically speaking. The main word of this sentence is シンシア, a noun, at the end, and everything else is a long modifier that modifies シンシア.
In Japanese, this type of sentence is called 体言止め【たいげんどめ】, a rhetoric device very often found in novels, screenplays, poems and lyrics (and sometimes in a news article). In English this kind of sentence is classified as a minor sentence (e.g., "Hello.", "The one on the right.", "At eight o'clock.").
So this sentence ending with a noun:
さっきから動物に餌を与えるのを楽しんでいるかのようなシンシア。
(literally) Cynthia who looks as if she were enjoying feeding an animal.
...conveys exactly the same information as a regular (or major) sentence with an obvious subject (topic) and a predicate:
シンシアはさっきから動物に餌を与えるのを楽しんでいるかのようだ。
Cynthia looks as if she were enjoying feeding an animal.
The difference is that the former sentence looks more vivid and impressive. When you encounter a 体言止め sentence like this in the middle of a novel, you don't have to translate it as a long noun phrase, because 体言止め is essentially just another method to make a regular sentence more interesting. You can safely treat it as if it were a regular sentence ("Cynthia looks ..."). It's not impossible to translate it as something like "Cynthia, the one who looks ..." if you like, but usually that's unnecessary.
Some songs and poems are almost exclusively composed of 体言止め lines/sentences. A classic example is ふるさと. A Japanese haiku often ends with a noun, too, although that's so common that almost no one call it 体言止め.
First of all,
「さっきから動物{どうぶつ}に餌{えさ}を与{あた}えるのを楽{たの}しんでいるかのようなシンシア」
is not a sentence. It is only a relative clause modifying 「シンシア」.
From the context provided, it is only natural to conclude that Cynthia is feeding another human ロエル.
「いるかのような」 is a key phrase here. If it were an animal, the author would not have used 「かのような」 ("as if to feed an animal").
"Cynthia, who seems as if she were enjoying feeding an animal for some time now."
A relative clause translated as one.
