You can use group_concat() function
SELECT l.first_name, l.last_name, l.id, group_concat(n.name), n.description
FROM
leads AS l inner join notes as n on l.id = n.parent_id
where l.deleted = 0
and n.deleted = 0
and l.id='104c4b25-adab-32f3-16ee-50d098a5dd5d'
group by l.id