It should be "a" because the sentence is introducing this personage to the reader for the first time. In later references to this "feminine sea personage", the writer should write "the personage" (or this/that or another determiner), and not "a".

Hi, You should use 'the' when talking about a specific item, for example: 'Look at the dog over there!' (You attention is on a specific dog) 'Thank you for the drink!' (you drank a specific drink) 'It this the right book?' (you are asking about a specific book) Use 'a' when talking about an item without meaning a specific example of that item, for example: 'I would like to have a dog.' (it could be any dog) 'I need a drink.' (it could be any drink) 'I want to read a book' (it could be any book) For your example of 'a feminine' or 'the feminine', it would be 'a feminine person', because she has not yet been made specific. But as soon as the attention of the reader is on her, she becomes specific. For example: 'A feminine person appeared' (now we have our attention on her) 'The feminine person looked around, and saw a masculine person watching her.' (now our attention is also on the masculine person) 'The masculine person looked back.'

It depends on what you want to say. If you are referring to a particular sea personage it should be “the”. If it’s not a particular sea personage, but one among other sea personages then it’s “a”.