Want to describe a characteristic of a person, in single word, "someone who readily takes responsibility/new tasks"
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You could say that he is proactive. From Wikipedia:
Proactive behavior involves acting in advance of a future situation, rather than just reacting. It means taking control and making things happen rather than just adjusting to a situation or waiting for something to happen. Proactive employees generally do not need to be asked to act, nor do they require detailed instructions.
It may not fit exactly, depending on the specific nuance you're looking for, but I'd say there's definitely an overlap in terms of attitude.
Self-starter comes to mind, though that is a compound word, not a single one.
If you could be more clear with how/where you are intending to use this description, it might help.
You can try the hyphenated compound Johnny-on-the-spot:
person who is on hand and ready to perform a service or respond to an emergency (Merriam-Webster)
Being ready to perform a task or duty right away, being there for something;reliable (Urban Dictionary)
A person who is available when needed, as in He always is there at the right time, a real Johnny-on-the-spot. (Dictionary.com)
1 A person or thing that initiates someone or something.
I wasn't sure if you wanted a noun or an adjective.
Ready and willing
Willing and able
Person with a can-do attitude
Never say never
Gets things done
A roll-up-his-sleeves person
You can say he has the initiative or he is an enterpriser.
enterprise (noun) the ability or desire to do dangerous or difficult things or to solve problems in new ways Merriam-Webster
an enterpriser (noun) a person who undertakes or engages in some enterprise; entrepreneur. D.com
- He looks like the right guy for that position. He has the initiative.
- Why don't you take the initiative and introduce yourself?
The term "eager beaver" means a person who is enthusiastic about performing work and frequently volunteers for tasks. It sounds a bit quaint, but does have the desired meaning.
A particularly enterprising person is sometimes called a go-getter.
an aggressively enterprising person (Mirriam-Webster)
a person disposed to initiate action, rather than take instructions; an enterprising person; a person with a strong drive to accomplish useful goals; especially, one whose career progresses rapidly. (Collaborative International Dictionary of English)
Volunteer - A person who offers to do something out of their own will.
You could say that a/that person takes ownership of the problem.
This doesn't fit your "single word" criterion, however, and simply calling him the problem's owner doesn't express it either, as it almost suggests the problem was his fault!
A person who steps into another's role could be seen as their successor, as stepping up, or as taking up the previous person's mission.