For the ideal egg texture, bring the water to a boil, then boil the 5 eggs for 9-10 minutes. Remove from the water and place directly into an ice water bath. Once cool enough to handle, carefully peel the shells off the eggs. Set peeled eggs aside.
Slice the eggs in half, then separate the yolks from the whites. Roughly chop the egg whites and yolks (separately) until they are mostly the equal in size, but still somewhat chunky.
In a bowl large enough for all of the eggs, add the mentaiko (scraped out of the sack if using) along with 1 tsp rice vinegar, 1 tsp sriracha, 1 tsp heavy cream, and 1/4 kewpie mayonnaise. Stir until fully combined. Taste and adjust flavorings as needed.
Fold in the chopped egg whites, then fold in the chopped yolks. Stir until well combined. Taste and adjust as needed.
Eat as is, with chips, or in between 2 slices of bread for a sandwich.