Just looking at the welds and frame geometry show this is a department store bike. I'm not sure if this bike is a true mountain bike, or a "mountain style" bicycle not designed for off-road use but made to look like it. For a department store bike, it looks decent, and if you like it, so much the better. I'm not aiming to disrespect your ride, just try to identify its origins, and it definitely appears from a department store.
Things I notice:
The welds of the rear suspension swingarm, come to an abrupt end with no shaping attempted, instead having a round piece of metal "endcap" brazed on the back ends with no tapering.
The suspension pivot punctures the middle of the down tube's girth, getting me concerned about stress cracks under heavy use unless that area is reinforced.
The riser bar instead of being one shaped piece are two brazed together.
The double crown front suspension -- not sure how good/bad it is.
The rear derailleur hanger is attached to a brazed-on semicircle of sheet metal... that is a major red flag. However, the hanger looks removable which is a plus.
The suspension geometry (and the rear shock) reminds me of the Trek Y series, or the Gary Fisher Joshua series -- a very good bike for the time (I have a Joshua F3 I'm keeping in mint condition until I feel I have the skill for it), but the geometry is outdated these days by newer bicycles. The pictured bike also looks a lot heavier than a Y or Joshua.
Lastly, and this is a personal thing, please replace that cable lock with a U-lock [1]. Even a Kryptonite Keeper will ward off would-be thieves who can easily snip through cable locks in seconds. It requires a thief with better tools to break a U-lock off.
[1]: U-lock or dedicated security chain (not the stuff that is cut to length at Home Depot.)