The iPhone 6s has the same sensor size as the standard iPhone 7 (not the two lens plus version) and that lens is just over 29.7mm so let's call it 30mm on a Full Frame 35mm camera. The crop factor of the K-70 is roughly 1.53x so 1.53x = 30; x = 19.6mm equivalent, which is around 20mm in round numbers.
The only Pentax prime available NEW at this focal length is the DA 21 Limited (@f/3.2 wide open). In this article
HD vs. SMC Pentax Limited Primes Review - Introduction | PentaxForums.com Reviews you can find this page:
HD vs. SMC Pentax Limited Primes Review - Bokeh | PentaxForums.com Reviews showing the bokeh of the SMC and HD versions of this lens. You can also look in sample galleries.
The other option of the HD DA 20-40 Limited is a good one. This very limited range zoom is a treat to own and shoot with. It is slightly faster and wider at 20mm (f/2.8 @ 20mm vs. 3.2 @ 21mm) but I have not seen detailed shots comparing the two for bokeh.
Faster options exist from other manufacturers as you may have noted earlier in the thread. These tend to be narrower in their field of view (24mm lens on the K-70 will be equivalent to about a 36-37mm on 35mm film giving a narrower view than the native lens on the iPhone. However these can be found with f stops up to f/1.4 which offers a shallower depth of field and more low light options and hopefully more pleasing bokeh but you'd have to review images to be sure.
Wider options exist in legacy manual focus and zoom lenses - but bokeh on wide angle lenses is typically not the most sought after feature. Most wide angle lenses are less optimal used this way due to the perceived depth of field in most shots.
For portraits longer focal lengths are "typically" preferred but no one will stop you using a wider perspective if you maintain a enough distance and don't try to get tight framing - or if you don't mind the extended nose and distorted features that a tightly framed headshot will give with that wide of a lens. On my K-3 I have used even the 15mm for a "portrait" of sorts and I have used 400mm as well. The most appropriate lens will depend on your framing and your desired working distance from the subject and the look you want to get. Conventionally on APSC that would be anything in the 50-135mm range which mimics the full frame 70-200 field of view that pros use a lot for portrait work. Many people are reluctant to the use the longer end and would suggest 50-85mm as the most comfortable prime lens range. (I'm not one of them, I'll try to use any length if I can get away with it! LOL)