Taking a one-way flight on Singapore Air to Bangkok that has a stopover in Narita and planning to get out during the layover (ultimate destination is Tokyo) since the flight was a lot cheaper. The layover is about 15 hours long, we land in Narita at 7pm and the plane leaves at 11 am the next morning. Will I be able to retrieve my checked baggage in order to prevent my luggage from going to Bangkok?
Unless you specially request it, you will not be able to retrieve your bags at Narita, since they will be checked through to Bangkok and transferred directly to the next plane. You will thus need to bring everything you need with you as carry-on luggage.
However! There is an option to request at check-in that they "short-check" your bags to Narita only. This is getting harder and harder to do, and is not guaranteed, but since you do have a nice long overnight layover of 15 hours, it's possible, and per anecdotal evidence Singapore Airlines should allow this.
Also, do you have a ticket out of Thailand? If you're flying one-way, you are likely to be asked for one at check-in, and may be denied boarding without it: Do I need a bus/train/plane ticket out of Thailand in order to get a Visa-on-Arrival?
Update: Corrected answer to note that short-checking may actually be a realistic possibility here.