NHF – new heroes for Heroes V 3.1.

20% is already quite a lot. You explore the map, look for human units, and attack them. If they are far from your castle, you summon them using a spell that you start with. I don't think the probability of getting the desired unit is low, around 12%. What's not very good is the lack of empathy. Therefore, instead of diplomacy, you can use the "Gather Troops" ability.

With proper development, the synergy with pirates will be very strong. And these are the units that you won't mind sacrificing.
They also have a good growth rate.

Nevertheless, the voting speaks for itself. You could try to rework Mirabelle's special ability in such a way that the ability is appealing and the summoned unit doesn't disappear quickly. To achieve this, the unit needs to be more durable, perhaps with a higher tier (5-6 tier with corresponding parameters). For balance, you can adjust the numbers in the formula for calculating the issuance of these units. If the units are durable, they will gradually accumulate, and by the late game, this will be a significant stack that might be worth not taking anyone else from the castle for.
I also like Fiona, but I repeat: "...the voting speaks for itself," and I don't see any other reasons for the dislike of Fiona.

Added 4 minutes later
By the way, within the lore of Ashan, there is a pirate unit that is tier 5 - the Elf Shaldan. You could repurpose this special ability for Vislot. Although I also like that pirate, of course.