Greetings from the Academy of S. Gabriel!
You asked about the name <Tess Boncheval> for a 1450s persona, and you mentioned that you had chosen that period because you like Shakespeare.
We'd like to begin by pointing out that Shakespeare lived from 1564 to 1616. If you are especially interested in setting your persona in his period, you will probably want to relocate it to the second half of the 16th century, in Elizabethan England.
The byname <Boncheval> 'good horse', though originally French, is found in England in 1212. [3, 7] Bynames of this type were quite common in both France and England, and several of them have survived as surnames. [1, 4] It is therefore quite possible that <Boncheval> might have survived into the 1450s or even into Elizabethan times, though our only actual citation is the one from 1212. Some additional support for this possibility is provided by the existence in 1601 of the surname <Chevell>, which is apparently from the byname <Cheval> 'horse'; like <Boncheval>, <Cheval> is attested from the early 13th century. [2, 4]
You are correct in thinking that <Theresa> didn't appear in England until after our period. Until the later 16th century it was confined to the Iberian peninsula, and it only spread with the fame of St. Teresa of Avila, who died in 1582.  Unfortunately, <Tess> is an English pet form of <Theresa>, so it's even further out of our period.
We didn't find any similar given names that that were in use in England in the 15th or 16th century. In the 12th and 13th centuries, however, there was a feminine given name <Tiece>, <Tiecia>, or <Tece> that had been brought to England by the Normans.  (The first two forms were probably pronounced roughly \TEET-seh\; the last was probably closer to \TATE-seh\.) <Tiece Boncheval> would be a fine feminine name for this period, say about 1200.
You may very well want to consider other possibilities for given names. You'll find quite a few on the Web at:
The first of these articles is a large list of feminine given names found in England in our period; however, most of these names are cited from the 12th through the 14th centuries. The second article contains both men's and women's names, but they are all from the second half of the 16th century.
Arval Benicoeur also contributed to this letter. We hope that it has been helpful. If you have any further questions, please don't hesitate to write us again.
For the Academy,
 Dauzat, Albert. Dictionnaire E/tymologique des Noms de Famille et Prénoms de France (Paris: Librairie Larousse, 1989). (The slash represents an acute accent over the preceding vowel.)
 Hitching, F.K. & S. Hitching. References to English Surnames in 1601 (Walton-on-Thames: Chas. A. Bernau, 1910).
 Jo:nsjo:, Jan. Studies on Middle English Nicknames, I. Compounds. Lund Studies in English 55 (Lund: n.d.). (<o:> stands for o-umlaut.)
 Reaney, P.H. & R.M. Wilson. A Dictionary of English Surnames (London: Routledge, 1991).
 Talan Gwynek. 'Feminine Given Names in _A Dictionary of English Surnames_', 1996.
 Withycombe, E. G. The Oxford Dictionary of English Christian Names (Oxford: Oxford University Press, 1984).
 We don't really know why someone would have been called 'good horse'. We can guess that he owned a particularly fine animal, but the nickname might just as well have been intended ironically and meant that he owned a really wretched nag. Perhaps he was a horse-dealer with a habit of advertising a horse as a <bon cheval>. Or the name may have meant something completely different; you can probably come up with other more or less plausible explanations of your own. As is very often the case, we can't be sure of the intended meaning of the nickname even though its literal meaning is perfectly clear.