It looks like some type of Rubus such as blackberry or raspberry. Is the stem prickly?

No there is almost no stem.

It's a Rubus. Either a seedling or a runner. The petioles are visibly prickly.

Plant has now grown up a little, it stem is prickly will you please look at it again. thanks Last year there was no such plant in garden bed, how did it grow by itself?

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Yes it’s still a Rubus and now looking like R idaeus. A Raspberry. It’s not hard to imagine a bird eating a berry in a neighbouring gardening and defecating in yours. Or if that is a fence behind the bed what’s growing on the other side? It does look a little too robust for a first year seedling. More like a runner.

So it is worth keeping and let grow next year to eat fruits? thanks

Not necessarily. Almost certainly not if it’s a seedling. If it’s a runner from next door find out the quality of their fruit.