Come on, Not even Happy considers Lissana as his mother, that role has led Lucy, even in the Fairy Tail Magazine made it clear that for Happy Lucy is much more important than Lissana
And even more, when Happy is happy or needs someone to comfort him, he always looks for Lucy
If one analyzes all the hints of the characters, realmenet it would be very difficult to get something from Lissana with Natsu, it is easier to find Hints of Natsu with Lucy, Natsu with Erza and Natsu with Wendy ... we go until juvia is more Close to Natsu that Lissana, the poor girl was tremendously relegated .... I think his biggest action is to have asked Lucy to never leave Natsu ... I think she is a Shipper more NaLu
Anyway, I think that even so it is very implicit the issue of the couples in the end, does not require much explanation, same is the matter that if you want to compare the Situation of her with that of Hinata and Orihime ... will be in a great Error because the work styles are very different
At the end the only one with Harem is Laxus