Tonkatsu sauce is crafted by stewing vegetables and fruits, then mixing in vinegar and sugar, salt, and spices.
While the sauce has its origins in English Worcestershire sauce, the flavor was changed to better suit the taste of the Japanese people, and it became its own original Japanese condiment, with a special sweetness and full-bodied flavor, sourness, and thickness.
The primary ingredients in Japanese sauce are vegetables and fruits, to which vinegar and many spices are blended.
This had the effect of deepening the richness and increasing the appetite.
Japanese tonkatsu sauce uses a lot of vegetables and fruits like tomatoes and apples, which make it a highly viscous sauce well suited for deep-fried foods, without getting them greasy or soggy.
This breaded meat dish and sauce made from vegetables, fruit, vinegar and spices pair well with the delicate Japanese staple food white rice.
Likewise, the sourness of the sauce gives heavy fried foods a taste that is refreshing and easy to eat.
Tonkatsu and its sauce are inseparable.
The thing that helped fuse the western "cutlets" with Japanese cooking the most is the sauce, and in fact it would not be a stretch to say that without the sauce, tonkatsu would not exist.
 
---Ingredients---
Water, high fructose corn syrup, sugar, distilled vinegar (made from alcohol), tomato paste, salt, rice starch, apple puree, yeast extract (contains soy & gluten), prune paste, spices (contains celery), carrots, onions, lemon juice.