Complex sentence: She compares the character with Mephistopheles, because a human needs to wager his soul with her in order to get the ultimate knowledge, however, the Mistress does not force anyone to abandon their moral values, and therefore "is not painted in dark colours".
Split into simpler sentences: She compares the character with Mephistopheles, because a human needs to wager his soul with the Mistress in order to get the ultimate knowledge. However she does not force anyone to abandon their moral values, and therefore "is not painted in dark colours".